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 Inc., 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 yen ($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 Inc., the …