TOKYO, Feb 17 (IFR) - Japanese government bond prices mostly inched lower on Monday, shrugging off disappointing economic data in very thin trading as the Bank of Japan began its two-day policy meeting at which it was expected to stay the course.

The yield on benchmark 10-year notes added half a basis point to 0.590 percent, but was still wallowing around its lowest levels since mid-November.

In mid-morning trading, the 20-year zone turned weaker, sending its yield up one basis point to 1.450 percent, ahead of Wednesday's monthly 1.2 trillion yen ($11.78 billion) auction of 20-year JGBs.

The BOJ did not offer to buy any superlong JGBs in the secondary market under its massive JGB purchase programme, instead just seeking a total of 900 billion yen of 1-year to 10-year JGBs in three tranches.

The BOJ is expected to stand pat at its meeting and maintain its commitment of increasing base money, its key policy gauge, at an annual pace of 60-70 trillion yen.

The JGB market had a muted reaction to news that Japan's economy grew less than expected in the fourth quarter of last year as consumer spending, business investment and exports disappointed in a worrying sign of waning momentum ahead of April's planned sales tax increase.

The 0.3 percent expansion in the October-December quarter fell short of the median estimate for a 0.7 percent increase, and matched 0.3 percent growth in July-September.

Lead March JGB futures moved in a narrow 144.95-145.03 range before finishing midday down 0.02 point at 144.98.