Christian Loamanu, a Tongan on Japan's national rugby squad, was suspended on Thursday after testing positive for marijuana, his club side said.

The 22-year-old winger, who played in the 2007 World Cup for Japan, denied using the drug and requested a second test after a positive result last month, Toshiba Brave Lupus said in a statement.

"The decision is temporary until the result of the sample B test comes out, but he has been suspended from the Microsoft Cup final between Toshiba and Sanyo Wild Knights this weekend," the team said.

It said it would kick Loamanu out the team if the second test came in positive.

The Japan Rugby Football Union said it would wait for the second test because Loamanu had protested his innocence.

"We are going to watch how it develops and make a proper decision later. I sincerely apologise that we caused such a disturbance for rugby fans and those concerned with the sport," JRFU vice chairman Noboru Mashimo said.

Marijuana is taboo in Japan, which strictly forbids both hard and soft drugs.

Japan's traditional sport of sumo has also been rocked by marijuana allegations, with three Russians ousted over use of the drug.

Police last week arrested a Japanese sumo wrestler, Shinichi Suzukawa, whose ring name is Wakakirin, after he was allegedly found with 16 grams (0.56 ounces) of marijuana.

The Japan Sumo Association dismissed Wakakirin but stopped short of slapping him with the heaviest punishment - expulsion from the sport - citing his age, a decision many Japanese faulted as too lenient.

Dismissal rather than expulsion means that Wakakirin can request a severance package of more than Y5 million ($A86,000). However, the sumo association said Thursday that Wakakirin had decided to forego it.

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