TOKYO (Reuters) -Japanese beverage group Suntory Holdings said on Tuesday that CEO Takeshi Niinami has resigned following a police investigation into his purchase of a supplement that may be illegal.

Niinami, an outspoken business leader who has often served as the face of corporate Japan at Davos and other international events, told the company he purchased the supplement believing that it was legal, Suntory said in a statement.

The Tokyo Shimbun daily reported earlier that police in Fukuoka prefecture have been conducting an investigation into whether supplements containing cannabis components had been sent to Niinami's home - a probe that is connected to a man who was arrested in July.

Suntory, best known overseas for its whisky and beer but which also makes a range of soft drinks such as Orangina-branded soda, said Niinami, who was also chairman, told the company he was the subject of a police investigation on August 22.

He resigned on September 1.

"He was a bold, decisive leader that got things done and I truly respected him," Suntory President Nobuhiro Torii told a press briefing in Tokyo, adding that Niinami was responsible for significantly expanding the company's revenues and profits.

"In that respect - and I told this to him yesterday as well - it's a real shame that we couldn't continue as a team."

According to Tokyo Shimbun, police questioned Niinami and searched his Tokyo home but no illegal drug possession or use has been confirmed.

A Fukuoka Police official was not immediately available to comment on the report.

Niinami, a fluent English speaker, is chair of the powerful Keizai Doyukai business lobby and has served as economic adviser to several Japanese prime ministers.

A graduate of Harvard Business School, he was previously chief executive of convenience store operator Lawson before becoming Suntory's president in 2014, the first head of the firm from outside its founding family.

Japan is known for its strict drug laws. Late last year, Japanese endoscope manufacturer Olympus Corp sacked then-CEO Stefan Kaufmann after an allegation that he had purchased illegal drugs.

In 2015, police arrested Toyota Motor executive Julie Hamp, an American, on suspicion of illegally importing the painkiller Oxycodone into the country. She was later released.