Amid revelations of forged official seals and tampering with records to make them appear old, expense-account padding has tarnished badly the Japanese bureaucrat's reputation for honesty and honor.

In a spreading scandal well-documented by reporters and auditors many government executives have been caught spending thousands of public dollars nightly in Tokyo's expensive restaurants to treat other officials and members of parliament from whom official favors might be expected.

Much of the money was obtained by falsifying travel vouchers for phony business trips, giving rise to a journalistic code name of "the empty-trip scandals."

Some of the devices were ingenious, as in the case of the employes of a quasi-government corporation who prepared "old" records by exposing ledger pages to sunlight and by splattering them with coffee stains.

So far, eight executives of two large government-supervised corporations have been forced to resign. Others have been punished and more heads are expected to roll.

To many Japanese, the revelations have been more shocking than the Lockheed case, which involves the alleged bribery of former prime minister Kakuei Tanaka.

Politicians come and go in Japan, but the bureaucracy remains to place with great power and has enjoyed for many years a reputation for dignity and trustworthiness. It is an elite class of well-educated mandarins whose probity is rarely questioned.

For more than four months, however, some of them have been ruthlessly exposed as petty cheats who, as part of office policy, squirreled away money from phony trips so they could wine and dine politicians and other bureaucrats who supervise their work.

In the most publicized case, the Japan Railway Construction Corp., a semigovernment company that builds bullet-train lines and tunnels, was revealed by government auditors to have misappropriated nearly $1.6 million by issuing vouchers for fictitious trips and other schemes over a 16-month period.

Much of the money was used to entertain officials of the ministries of transport and finance, both of which supervise the corporation's budget and construction plans, and to buy tickets to influential politicians' fund-raising parties. Five top officials, including the president, have been dismissed.

The scandal broke initially through the investigations of a 10-member Asahi newspaper reporting team headed by Hisamitsu Tani and Shunji Taoka. Their first story focused on the railway corporation and the fake trips. After it was published, they were flooded with anonymous tips of similar malfeasances in other ministries and quasi-government corporations.

More than 500 letters have poured into Asahi, many of them from veteran, high-ranking bureaucrats describing their own cheating. "They were simply conscience-stricken," reporter Taoka said.

The Asahi team also obtained a ledger of accounts from one of the Akasaka district's most prestigious geisha houses, where the average cost per guest ran to about $185. To treat 14 officials, including some from the Ministry of finance's budget bureau, the railway company paid out about $2,500-for one two-hour dinner.

The headlines were still hot on the railway scandals when a new tale of high living and gift-giving broke over still another quasi-government corporation: The International Telephone and Telegraph Co., known as KDD, a government-protected monopoly that handles all overseas telephone and cable business.

It was reported that four or five KDD executives were spending about $40,000 a month at a single night club in the Shinjuku section of Tokyo and charging it off as company business. A tax official estimated that the company had spent more than $20 million in five years on entertainment.

A more serious account surfaced of attempted smuggling of foreign gifts into the country by telegraph company executives in the president's office. Two presidential assistants were caught at the new Narita International Airport outside Tokyo with about $40,000 worth of undeclared articles, including jewelry.

Official accounts now suggest that the smuggling was widespread, involving 20 different trips into Japan with more than 4,000 foreign-bought luxury items.

The furor over KDD expense account operations was particularly loud because the company charges what many consider extraordinarily high rates for overseas cable and telephone traffic. The company is now planning to reduce the rates.