TOKYO, Nov 30 (Reuters) - Japanese government bond prices were flat to slightly weaker on Monday, as investors stayed on the sidelines ahead of 10-year and 30-year bond auctions later this week. Investors also refrained from making big moves as they cautiously await the Japanese government to compile the budget plan next month. The Bank of Japan kept the size of its bond purchases unchanged on Monday, buying 420 billion yen of three-to-five year JGBs and 30 billion yen of 25-40 year JGBs. Benchmark 10-year JGB futures fell 0.05 point to 152, with a trading volume of 12,979 lots, while the 10-year JGB yield rose half a basis point to 0.030%. Yields were flat at the super-long zone, with the 20-year JGB at 0.395%, the 30-year note at 0.650% and the 40-year JGB at 0.700%. At the shorter end of the curve, the two-year JGB yield was unchanged at minus 0.130%, while the five-year yield added half a basis point to minus 0.105%. Japan’s finance ministry will offer 2.6 trillion yen of 10-year JGBs on Tuesday followed by 900 billion yen of 30-year notes on Wednesday. (Reporting by Tokyo markets team; Editing by Vinay Dwivedi)