The city's top investigator said in a broadcast interview yesterday that his department is investigating contributions by the taxi industry to Mayor Edward I. Koch's 1985 reelection campaign.

"We have opened an investigation into that matter and have confirmed publicly that we are examining specifically the question of the filings," said Investigations Commissioner Kenneth Conboy.

He said the question is "whether the election law requirements for full disclosure were complied with."

U.S. Attorney Rudolph W. Giuliani said he does not think Koch is involved in the corruption scandals in the Taxi and Limousine Commission and the Parking Violations Bureau.

Tom Kelly, a spokesman for Koch, said the investigation is "only right, in light of the investigations. He [the mayor] is not worried."

The taxi commission, which regulates the city's metered yellow cabs, has been under state and federal investigation in the awarding of lucrative medallions.

Commission Chairman Jay Turoff resigned March 10, but has denied any wrongdoing.

The New York Times reported yesterday that federal prosecutors are focusing on a potentially larger corruption case involving Turoff and other political figures and free taxi medallions that brought a fleet operator $20 million in gross receipts in five years.