TOKYO, Sept 2 (Reuters) - Investors in Japanese government bonds on Wednesday refrained from placing big bets ahead of a speech by Yoshihide Suga, the frontrunner to replace Prime Minister Shinzo Abe, and on caution due to the auction of a 30-year government debt later this week.

Suga, currently the chief cabinet secretary under Abe, is expected to announce his candidacy and explain some of his policies later in the day. The largest faction of the ruling party has already thrown its support behind him.

Investors also avoided taking big positions ahead of an auction of 30-year government debt on Thursday, which will be an important test of demand for longer-dated paper.

Benchmark 10-year JGB futures rose 0.05 point to 151.75, with a trading volume of 16,299 lots.

The 10-year JGB yield was flat at 0.040% and the 20-year JGB yield was also flat at 0.420%.

At the long end of the curve, the 30-year JGB yield was flat at 0.610%.

The five-year yield was flat at minus 0.080%.

At the short end, the two-year JGB yield was flat at minus 0.120%. (Reporting by the Tokyo markets team; editing by Uttaresh.V)