Japan Airlines says one of its pilots cheated two years ago on a preflight breath test by asking somebody else to do it in his place.
The company said a 59-year-old captain got his copilot to take a test for him before their flight from Japan's Narita Airport to Chicago in December 2017.
The captain was on duty after the test, as test results were recorded without being checked by staff other than pilots at the time.
The misconduct was later revealed when the copilot reported it to the company.
Airline officials said the captain told them he used a stand-in because his alcohol level was nearly the firm's limit when he used an auxiliary test device before the actual check.
Last month, the transport ministry gave JAL a business improvement order following a series of alcohol-related problems involving its pilots.
But the firm has not reported the captain's wrongdoing to the ministry.
The officials said the preliminary figure the captain declared did not exceed the firm's alcohol limit so they did not think it was necessary to report the case.
But they added that having somebody else take a breath test is illicit so they should have reported it.