TOKYO, Jan 5 (Reuters) - Japanese government bond yields were flat on Wednesday as solid demand for 10-year notes at an auction boosted sentiment, which was earlier dented by firmer U.S. Treasury yields overnight. The 10-year JGB yield was flat at 0.085% and the 20-year JGB yield was flat at 0.485%. The government auction for 10-year debt attracted bids worth 3.46 times the amount sold, higher than a bid cover ratio of 3.16 times at the previous auction. U.S. Treasury yields for most maturities rose overnight, as bond investors geared up for interest rate hikes from the Federal Reserve by mid-year to curb the stubbornly high inflation. The 30-year JGB yield rose 0.5 basis point to 0.695%, while 40-year bonds were not traded and their yield stayed at 0.745%. The two-year JGB yield was flat at minus 0.090% and the five-year yield was flat at minus 0.070%. Benchmark 10-year JGB futures fell 0.04 point to 151.39, with a trading volume of 23,265 lots.