Japan's largest manufacturer of computer memory chips and two major Japanese auto makers yesterday increased their U.S. prices, saying that the falling dollar has reduced profits from sales in the United States. Other Japanese automobile and semiconductor makers are expected to follow suit.

NEC Corp., Japan's top maker of semiconductors, also cited a recent spate of unfair-trade charges by American competitors as another reason for its 20 percent price increase. U.S. companies have accused the Japanese of dumping chips in the United States: selling them at less than the cost of producing them to gain a large share of the U.S. market.

The price increases are a direct result of an agreement last September among leading industrialized nations to lower the value of the supercharged dollar, which has been a major factor in the record U.S. trade deficit. Since February, the dollar has fallen 18 1/2 percent, with 7 percent of that drop coming in the past two months since the agreement for coordinated action to lower the value of the dollar in relation to other currencies.

The dollar took its greatest fall against the yen. Before the September meeting of finance ministers of the United States, Japan, Britain, France and Germany, one dollar was worth 242 yen. It dropped under 200 yen to the dollar last week for the first time in 20 months, but bounced back to sell yesterday on the Tokyo exchange for 202.70 yen to the dollar.

An overvalued dollar sucks in a flood of imports by making foreign goods less expensive. It also hurts U.S. export sales by increasing the cost of American products overseas.

Nissan Motor Co. and Honda Motor Co., Japan's second- and third-largest auto makers, said they are raising prices by about 4 percent. Japan's largest car company, Toyota Motor Corp., said it also is considering an increase. The price increase will amount to an average of $403 on a 1986 Honda, that company said.

Although the Japanese companies were quick to increase prices to reflect the weakening of the dollar, U.S. officials of Honda and Nissan acknowledged yesterday that they had not cut prices as the dollar rose, preferring to retain the extra profits gained in the currency exchange.

Auto analysts here don't expect the price increase to have any impact on sales of Japanese cars in the United States. "They are just trying to maintain their profit margins. This is the only place where they are making money," said James Harbour, a Detroit-based auto industry analyst.

"My own reaction is, what kept them" from increasing their prices? asked David Healy, an auto analyst with the Wall Street firm of Drexel Burnham Lambert Inc. "I would not be surprised to see more price increases."

He added that demand for Japanese cars remains high and that any increase in prices by the manufacturers still would leave them lower than when import restraints were in force and dealers were tacking on surcharges as high as $1,500.

In the high-tech field, other major Japanese semiconductor makers, including Fujitsu Ltd. and Hitachi Ltd., are expected to follow NEC's lead.

U.S. manufacturers, who have filed three unfair-trade cases against the Japanese, declined comment on the NEC price announcement. The U.S. Trade Representative's Office also is pursuing trade action against the Japanese.