An industry group is said to have paid employees of trucking firms who attended a campaign meeting of transport minister Saito Tetsuo before last month's Lower House election.
A trucking association in Hiroshima Prefecture has admitted that its related policy study group made the payments.
The group reportedly asked member firms of the association to send employees to Saito's campaign speech meeting at a hotel in Hiroshima City, and paid them in cash in the name of travel expenses.
Saito, a veteran lawmaker of the ruling coalition partner Komeito, was elected to the chamber from the Hiroshima No.3 constituency in the October 31 contest.
The main governing Liberal Democratic Party did not field a candidate in the constituency because the LDP legislator who previously held the seat had been convicted of buying votes.
The trucking association has not disclosed the amount of the payments or the number of people who received them.
A senior association official, Morii Shigeto, says the cash was intended as travel expenses and was appropriate. He denied there was any intention of vote-buying.
Saito said on Wednesday that neither he nor his office had any knowledge whatsoever of the travel expense payments. He said the association's related group should fulfill its accountability.
Komeito's Vice Representative Kitagawa Kazuo said such payments during the campaign period are not appropriate and are problematic.
But he said he understands that both Saito and his office had no knowledge of the payments, and the trucking association is going to offer an explanation.
Kitagawa said there's no need to discuss whether Saito should resign as minister because he knew nothing about the matter.