* Superlong tenor erases early slight losses and edges higher * 10-year yield remains in 0.8 pct-0.90 pct range * 10-year futures end up in relatively thin trade By Lisa Twaronite TOKYO, July 17 (Reuters) - Solid demand at a sale of five-year notes helped push up Japanese government bond prices on Wednesday, though moves were slight amid caution ahead of testimony from U.S. Federal Reserve Chairman Ben Bernanke later in the day. Investors will be watching Bernanke's testimony to Congress on Wednesday and Thursday for clues on when the U.S. central bank will begin to taper its $85 billion a month bond-buying programme. The Fed chief's continued reassurance that U.S. monetary policy will stay accommodative would keep a lid on U.S. Treasury yields, and in turn on JGB yields. "Those kinds of external factors will be this week's theme, after today's 5-year auction is done," said Maki Shimizu, senior bond strategist at Citigroup in Tokyo. The benchmark 10-year yield was flat at 0.820 percent after wobbling between 0.815 percent and 0.825 percent, still within the range of 0.80 percent and 0.90 percent in which it has traded since late May. The 10-year JGB futures contract ended up 0.11 point at 143.22 after rising as high as 143.26. Volume was a relatively low 16,346 contracts, though higher than Wednesday's volume of 12,278 contracts, which was the lowest since Dec. 25. The five-year yield fell 1 basis point after the auction results were announced to 0.290 percent, matching a four-week low touched last week. The superlong tenor erased its early slight losses, with the 30-year yield falling 1.5 basis points to 1.835 percent. The 20-year yield slipped 1 basis point to 1.710 percent. The Ministry of Finance offered 2.7 trillion yen ($27.19 billion) of newly issued five-year notes with a coupon of 0.3 percent, below the 0.4 percent coupons at the past two sales. The notes sold at the lowest price of 100.01, in line with most market expectations, and drew bids of 4.28 times the amount offered - slightly down from the previous sale's bid-to-cover ratio of 4.36 times, but still strong. The tail between the average and lowest accepted prices came in at 0.01, shrinking from 0.02 at last month's offering. A smaller number means stronger demand for the bonds. "People thought the sale would be okay, or at least decent, and it turned out to be maybe slightly better than expected," Citigroup's Shimizu said. "But whether there will be demand below 0.3 percent, that is the question. I'm a little bit sceptical as to whether this rally can continue below that level." According to International Financing Review, a Thomson Reuters publication, one major Japanese bank seems to have bought around 400 billion of the new 5-year notes, probably to sell part of them to the Bank of Japan under the central bank's operations for its massive asset-buying stimulus. Minutes of the BOJ's June policy-setting meeting released on Wednesday showed some board members proposed offering longer- dated fixed-rate funds in its market operations to curb excessive interest rate fluctuations, but others said the measure could be misinterpreted as a change to the bank's monetary policy framework. At that meeting, the BOJ left monetary policy unchanged as widely expected, maintaining its pledge to expand the supply of money at an annual pace of 60 trillion to 70 trillion yen to achieve its 2 percent inflation target in two years.