AN AMERICAN HAS an easy time visiting Japan these days. Indeed, this land of blue jeans, rock music, ice-cold vending machines and automatic doors -- not to mention McDonald's, Wendy's, Mr. Donut, and the like -- feels so familiar that you have to be pinched once in a while and reminded you are in the midst of a unique and ancient culture.

I had only one uncomfortable experience during a recent stay in Japan lasting several weeks, and that was in Hiroshima.

The new Hiroshima, the one that was constructed out of the ashes beginning in the 1950s, is a busy, crowded, ceaselessly noisy place. It has good restaurants and fancy department stores, but few of the trees and parks and other outdoor amenities of which the Japanese are so proud elsewhere.

There is one green, restful spot in the middle of the city, the triangular-shaped "Peace Memorial Park," built near the hypocenter of the world's first atomic bomb explosion on Aug. 6, 1945. I went there on a hot afternoon in late July, soon after arrival on the bullet train from Kyoto. It's easy to find in an understandably obsessed city: you just walk along Peace Boulevard until you cross Peace Bridge and come to the Peace Fountain.

It was eerily quiet when I got to the park. A vagrant dozed in the sun at the entrance. School children filed silently out of the Peace Memorial Hall, where they had just seen a film. Too late for the museum, I wandered along the paths, past the Peace Bell, the Peace Clock, the gravestones with Japanese inscriptions, the arc-shaped cenotaph with its eternal flame. All of this is neat and orderly and pretty -- too pretty for the horror it commemorates.

The main attraction is the "A-bomb dome," the only building still standing in the same condition it assumed at that moment 38 years ago. Until the bombing it was a sort of regional promotion office; ever since, it has been a stark shell of steel surrounded by concrete rubble. It is enclosed by a fence now and, remarkably to an American eye, utterly free of graffiti or defacement. No piece of modernist sculpture could make its point so dramatically.

The silence of the early evening was interrupted only by the sound of machinery -- earth-moving equipment that was carrying out some last-minute changes, some further prettying-up for this year's anniversary ceremony, when tens of thousands of people would converge on Hiroshima for an annual ritual of speeches, prayers and protests.

Gradually I began to feel awkward and out of place in the park. It was not that I was the only American there. Two GIs arrived, with a tripod and many cameras, to take pictures of the dome through the space in the cenotaph. A tourist couple sat off to one side, eating a picnic supper on a park bench.

But I became convinced that I was being singled out and eyed suspiciously, glared at by Japanese construction workers and by individual men and women strolling through the park and pausing for reflection. Not the ordinary curious glance a foreign tourist can expect almost anywhere, but a penetrating stare. It was as if my presence was an insult. All at once, I felt responsible. I wanted to cry out: "Listen, I'm sorry -- I really am. But you must understand. . . . I had nothing to do with this. . . . I wasn't even 2 months old at the time. . ."

It was a foolish impulse, of course, an irrational desire to convey a rational message. Most of the people around me would not have understood the language or the message. I left the park immediately and walked quickly back to my hotel.

The next morning, after a poor night's sleep, I went back. There was a line to get into the Hiroshima Peace Memorial Museum even before opening time. In the midst of a waiting crowd of Japanese tourists was one American family -- a man, a woman and their teen-age daughter. We gave each other a faint, perfunctory smile of recognition and shuffled along up the stairs to the entrance.

Inside the long, thin museum on stilts, Japanese ingenuity has assured a powerful impact in a tiny space. The visitor is bombarded with exhibits on the effects of the atom bomb's heat, its blast and its radiation.

A scale model of Hiroshima as it was in 1945 demonstrates its utter devastation. In a gruesome diorama, women and children inch along in confusion, their skin dripping from their bodies. There are photographs of people in tattered clothes, huddling together after the explosion, trying to figure out what to do. Then there are the steps that once belonged to the Sumitomo Bank, forever engraved with the shadow of a person who happened to be sitting there at 8:15 that morning.

Museum displays explain how the B-29 Enola Gay happened to make its fatal pass over Hiroshima just when it did. (An "all- clear" had sounded earlier that morning, and so the immediate casualties were even greater than they otherwise might have been.)

But there was something wrong here, too. I began to feel uncomfortable in a different way from the night before. The museum chronicles the fate of Hiroshima as if it occurred in a vacuum -- as if no war had been taking place at the time, as if there had been no Pearl Harbor. It presents without qualification the argument that American planners sought to maximize civilian casualties, but it does not even acknowledge the argument that the atomic bombs on Hiroshima and Nagasaki actually saved American and Japanese lives and, by ending the war, headed off mass starvation in Japan.

Again I felt the impulse to shout my feelings: "Listen, this isn't quite right. Yes, it is true that my people did this to your people, but, horrible as it was, you must understand the context . . ." But again I stopped myself; my confused message would not have gotten through, and I'm not sure that I would have believed it myself.

I went back outside. I wiped my eyes. I thought for just a moment about how tiny the Hiroshima bomb was compared to the nuclear weapons of today. Then I changed the subject of my thoughts, and went about my business.