TOKYO (AP) _ In its first advertisements since a crash that killed 520 people last August, Japan Air Lines declared in a New Year message Monday that it would work to regain public confidence.

''As an airline representing Japan, Japan Air Lines will once again proceed steadily, step by step, to become (a symbol of) 'trusted wings,''' said a full-page advertisement carried in 14 Japanese newspapers, including five nationwide dailies.

Last August, in history's worst single-plane disaster, a jumbo jet flying from Tokyo to Osaka lost a large part of its tail section, went out of control and crashed into a mountain. Only four of the 524 people aboard survived.

In November, the airline acknowledged that one of its pilots accidentally flew his jet dangerously close to sensitive Soviet air space near the area where the Soviets shot down a Korean airliner two years ago. JAL's top officials resigned following the August crash, a customary move for leaders in Japan following major disasters or scandals in their companies or departments.

''Japan Airlines has embarked on a new beginning under new managment,'' said the ad.

It said the airline would tighten its maintenance and put great efforts into its transformation from a semi-governmental to a private firm. In December, the government ended JAL's monopoly of international flights and opened the way for more competition from other Japanese airlines.

''The main advantage of privatization is the resulting healthy competition and a mutual increase of safety and convenience,'' the ad said.

JAL President Susumu Yamaji referred to 1985 as ''a year of great sorrow'' in a New Year address to employees.

''With this experience deeply etched in our minds, let us turn over a new leaf on the occasion of the new year,'' he said. Excerpts of his speech were broadcast nationwide by the Japan Broadcasting Corp.

The number of passengers on JAL's Tokyo to Osaka flights plunged 41 percent in September, the month after the crash, and since then, the number of passengers on all JAL domestic flights has been consistently lower than the previous year. In December, the number of domestic passengers was approximately a fourth less than in December 1984, said company spokeswoman Tomiko Yoshizawa.