TOKYO, Sept 15 (Reuters) - Japanese government bond yields were mostly lower on Friday as better-than-expected demand seen at two auctions this week boosted investor sentiment, while concerns lingered about a possible tweak to the Bank of Japan's (BOJ) ultra-easy policy.

The 10-year JGB yield JP10YTN=JBTC fell 0.5 basis point (bp) to 0.700%. The 20-year JGB yield JP20YTN=JBTC fell 0.5 bp 1.425%.

"Investors sentiment was lifted after auctions for 5- and 20-year bonds this week witnessed firm demand," said Naoya Hasegawa, a senior bond strategist at Okasan Securities.

"But caution abut a possible BOJ policy tweak still weighs on sentiment. Investors were actively buying the securities."

The 20-year bond auction held on Thursday saw healthy demand, despite a surge in yields across maturities after a report signalled a possible end to the BOJ's rate policy.

Earlier in the week, the auction for five-year also drew higher-than-expected demand.

The five-year yield JP5YTN=JBTC was flat at 0.275%.

The 30-year JGB yield JP30YTN=JBTC fell 0.5 bp to 1.670%.

The two-year JGB yield JP2YTN=JBTC rose 0.5 bp to 0.030%.

The 40-year JGB yield JP40YTN=JBTC rose 0.5 bp to 1.820%.

(Reporting by Junko Fujita; Editing by Subhranshu Sahu)

((junko.fujita@thomsonreuters.com;))