* Q4 oper profit 60.2 bln yen vs 30.9 bln yen yr ago
* Tapping growth in China, recovery in U.S. market
* Sees oper profit up 37 pct this yr at 305 bln yen
* Says impact of earthquake on demand is unclear
* Shares up 10 pct since quake (Recasts to lead on quarterly earnings, adds details)
TOKYO, April 27 (Reuters) - Komatsu Ltd , the world's No.2 construction machinery maker, doubled its quarterly operating profit and forecast further gains this year as it taps fast-growing demand in China and sees a continued recovery in the United States and Europe.
Komatsu, which trails U.S.-based industry leader Caterpillar Inc , said it was still unclear what impact the massive earthquake and tsunami that struck northeast Japan on March 11 would have on its earnings.
Komatsu is seen as a natural beneficiary of the rebuilding, and its shares have risen 10 percent since the disaster. The benchmark Nikkei average is down 6 percent in that period.
"It is unclear how much rebuilding-related demand there will be and we have not incorporated any into our forecast. We are looking for domestic demand to be flat year-on-year," Chief Financial Officer Mikio Fujitsuka told a briefing.
Komatsu posted an operating profit of 60.2 billion yen ($738.5 million) on Wednesday for the January-March quarter, up from a 30.9 billion yen profit in same quarter last year.
For the current year to March 2012, Komatsu forecast operating profit would rise 37 percent to 305 billion yen, bigger than the Thomson Reuters Starmine SmartEstimate of 294.7 billion yen based on predictions by 12 analysts.
The SmartEstimate puts a larger weighting on recent estimates by highly rated analysts.
Komatsu said it expected solid demand for mining equipment, given active investment in commodities production.
Over the past few years Komatsu has significantly boosted its presence in China, where aggressive spending on housing and infrastructure projects is driving up demand for wheel loaders, hydraulic excavators and mining equipment.
Komatsu said sales in its core construction machinery division rose by more than a third in China in the past business year, which ended on March 31. It also enjoyed gains in Europe and North America, with sales up 34 percent and 26 percent respectively.
Komatsu said quake-related factors depressed its operating profit by 7 billion yen in the past financial year.
It said the factories it shut after the quake were back up and running normally and that it had enough inventory of parts to get through the April-June quarter. Whether semiconductor output returns to normal is seen as a potential risk after June.
"Semiconductors will be a key factor. But we don't see it as a big risk as long as chipmakers are able to achieve a recovery in production according to their present plans," Fujitsuka said.
Prior to the announcement, shares of Komatsu rose 0.6 percent to 2,762 yen, against the Nikkei's 1.4 percent gain. (Reporting by Yuka Obayashi and Nathan Layne; Editing by Edmund Klamann and Michael Watson)