The transport ministry on Monday issued a strict warning to Japan Airport Terminal over its subsidiary's inappropriate transactions that benefited a consultant firm headed by the eldest son of Makoto Koga, former secretary general of the ruling Liberal Democratic Party.

The ministry summoned Kazuhito Tanaka, president of the operator of Tokyo's Haneda Airport, and gave the warning over misconduct related to a massage chair business in a terminal building at the airport.

The ministry said it is "regrettable" that a company responsible for public infrastructure has been involved in long-term misconduct at the airport terminal.

It urged the company to revise its compliance audit system and continue to report to the ministry on the status of measures to prevent a recurrence.

On Friday, a special investigation committee set up by the company announced that the inappropriate transactions had continued for many years, led by former company president Nobuaki Yokota and tolerated by former chairman Isao Takashiro. Yokota and Takashiro resigned the same day.

Following the ministry's warning, Japan Airport Terminal said that it will make sincere efforts to prevent a recurrence and regain trust.