TOKYO, May 2 (Reuters) - Japanese government bond prices rose on Monday, sending the benchmark 20-year yield to a record low, after Japanese share prices fell more than 3 percent to three-week lows on the yen's gains.

The 20-year JGB yield fell to as low as 0.235 percent , below its previous trough of 0.245 percent touched on April 20. It last stood at 0.240 percent, down 5 basis points from the previous session on Thursday.

Japanese financial markets were closed on Friday and will be shut again from Tuesday to Thursday for various national holidays.

The 10-year JGB yield dropped 3.0 basis points to minus 0.115 percent, edging near its record low of minus 0.135 percent.

The yield curve is flattening sharply on the view that investors purchase will concentrate on the longer end of the yield curve that still has positive yields.

The spread between 10- and 20-year yield shrank to 35.5 basis points, the tightest since June 2007.

Japanese shares tumbled on Monday as the yen extended its gains following the Bank of Japan's surprise decision not to expand its stimulus last Thursday.

The Nikkei average fell 3.1 percent as the yen hit an 18-month high of 106.14 per dollar.

(Reporting by Tokyo Markets Team; Editing by Kim Coghill)