* Superlong yields hit 3-week low
* Fall in shares prices helps JGBs
* Short-term yields at historic lows on BOJ easing expectations
* Shortage of short-term bonds leads to irregularity in BOJ buying
TOKYO, Feb 13 (Reuters) - Japanese government bond prices were mostly firmer on Wednesday, with the 30-year yield hitting a three-week low after a reasonably well bid auction of 40-year JGBs.
Short-term bond yields were stuck at historic lows as investors expect the Bank of Japan to step up buying in them to pump more money into the economy.
"Given strong expectations of bold monetary easing, short-term yields are likely to stay at current low levels for some time," said Katsutoshi Inadome, strategist at Mitsubishi UFJ Morgan Stanley Securities.
Market players had been optimistic about the quarterly auction of 400 billion yen ($4.3 billion) 40-year bonds, the longest maturity offered by Japan at the moment, pushing down their yields in the past couple of days.
While some market players said the actual auction result was not as strong as expected, on the whole the auction result was perceived as decent, boosting the entire "superlong" sector -- such as 20- and 30-year bonds.
The yield on 30-year bonds dipped 1.5 basis points to 1.955 percent, its lowest level in three weeks while the 20-year yield fell 1.0 basis point to 1.750 percent , also a three-week low.
The 10-year JGB yield fell 0.5 basis point to 0.740 percent , its lowest since Jan. 31, having peaked at 0.905 percent earlier this month. Its Jan. 25 low of 0.720 percent is seen as a near-term resistance.
The 10-year JGB futures price rose 0.08 point to 144.38 .
The bond market was also helped by a drop in Japanese shares, after an official from the Group of Seven cited concerns about excessive movements in Japan's currency, which pushed the yen up and prompted some selling in the Nikkei.
Yields on short-term notes were mostly flat, though they are already at historic lows.
The five-year yield was flat at a record low of 0.135 percent, on expectations that the successor to current BOJ chief Masaaaki Shirakawa will take drastic easing steps.
Prime Minister Shinzo Abe is expected to name a sympathiser of his reflationary policy agenda for the post later this month.
With Shirakawa seen as a lameduck governor, many market players expect the BOJ to keep its policy on hold at its two-day policy meeting ending on Thursday.
Still, some market players see a chance that the central bank will expand the target of its bond buying programme -- something seen as inevitable in the long run.
"You can't rule out the possibility that the BOJ will tweak its operations... It could consider diverting some of its bond buying to the 3- to 5-year zone from the current 1- to 3-year sector," said Akito Fukunaga, chief rates strategist at RBS.
A source of such speculation was an unusual stance in the BOJ's asset purchases lately. Although the bank has committed itself to buying 10 trillion yen of bonds in January-June, or about 1.7 trillion yen a month, it has not done any buying so far this month.
Market players say that is likely to reflect concerns at the BOJ that its buying could exacerbate a severe shortage of short-term bonds in the market.
The two-year yield hit a 10-year low of 0.025 percent last week mainly as brokers' desperate short-covering added to expectations of further BOJ easing, traders said.
This has prompted many market players to take the view that the BOJ will need to buy longer maturities to pump funds into the money market if it doesn't want to squeeze the short end of the bond market.
RBS' Fukunaga recommends flattening trade between two- and five-year maturities to benefit from such a move.