TOKYO, March 25 (Reuters) - Japanese shares slipped on Friday after eight straight sessions of gains, with heavyweights SoftBank Group and Fast Retailing leading the retreat.

The Nikkei share average had fallen 0.17% to 28,082.18 by the midday break, but was set to gain 4.6% for the week. The broader Topix inched down 0.12% to 1,979.22, but was on course for a weekly rise of 3.69%.

"Some investors sold shares to book profits after sharp gains in the past several sessions," said Ikuo Mitsui, a fund manager at Aizawa Securities.

"But the market is still relatively cheap, so some investors bought shares, which limited the losses."

Uniqlo clothing shop owner Fast Retailing and technology investor SoftBank Group dragged down the Nikkei the most, falling 0.96% and 1.93%, respectively. Phone company KDDI slipped 2.33%.

Banking giant Sumitomo Mitsui Financial Group fell 1.27% after executives at its brokerage unit SMBC Nikko Securities were arrested and indicted for alleged market manipulation.

Chip making equipment maker Tokyo Electron rose 0.62%, helping the Nikkei cap its slide, while drug maker Daiichi Sankyo jumped 3.67%.

Fanuc rose 1.12% after the robot maker announced a share buyback programme.

There were 113 advancers on the Nikkei index against 104 decliners.

The volume of shares traded on the Tokyo Stock Exchange's main board was 0.63 billion, compared with the average 1.38 billion in the past 30 days. (Reporting by Junko Fujita; Editing by Subhranshu Sahu)