Japan Airlines posted sharp increases in profits and sales in April-September period of 1997 due to a favorable domestic market, while the two other major airlines reported profit drops, according to earnings reports released Thursday by the three carriers.

JAL reported pretax profits of 21.5 billion yen in the first half of the business year, up 322.4 percent from the same period last year. Sales grew to 633.1 billion yen, up 5.5 percent.

A JAL official attributed the good performance to the airline's skillful use of the discount fare system, an attractive frequent-flyer program, a boost in the number of domestic flights and extended depreciation terms for its aircraft.

In contrast, All Nippon Airways Co. and Japan Air System Co. reported sharp drops in pretax profits for the April-September period, despite increased sales. ANA reported pretax profits of 13.5 billion yen, down 24.4 percent from the the same period last year, and sales of 467 billion yen, up 3.6 percent.

An ANA official said the domestic-oriented airline performed poorly in its operation of the discount fare system, particularly in the first quarter. Typhoons in September in Kyushu also were a factor, the official said.