The transport ministry conducted an on-site inspection Thursday of Yamato Holdings Co. after it admitted to overcharging corporate clients an estimated ¥3.1 billion over the past five years.

Six officials of the Land, Infrastructure, Transport and Tourism Ministry were seen entering the headquarters of a Yamato subsidiary. The moving company, Yamato Home Convenience Co., may face administrative punishment as a result of the inspection.

Yamato Holdings said in late July that the subsidiary had inappropriately billed an extra ¥1.7 billion for its moving services over roughly two years through June, affecting 2,640 companies.

It later said an internal probe had uncovered an additional ¥1.4 billion of extra charges in the past five years.

The practice occurred at 123 of Yamato Home Convenience's 128 offices, according to the ministry.

The overcharging included billing customers based on initial estimates, even in cases when the actual cost of a move was lower. Some employees have said the overcharging was aimed at improving business results.

Yamato Holdings plans to reimburse customers who were overcharged.