The ever-expanding graft case linked to last year's Tokyo Olympics has shed light on an oligopolistic structure of the Japanese advertising industry controlled by Dentsu, with a smaller rival implicated after allegedly trying to win its favor.

Haruyuki Takahashi, a former executive of the ad industry giant, has been served arrest warrants four times as he is accused of taking nearly ¥200 million ($1.3 million) in bribes as a member of the now-defunct Tokyo Olympic organizing committee.

Among those suspected of bribing Takahashi is Shinichi Ueno, the president of ADK Holdings, the country's third-largest ad agency. Ueno resigned as ADK Holdings president after he was arrested by Tokyo prosecutors earlier in October.