* Mazda Q1 profit jumps 54 pct
* Fuji Heavy books highest-ever Q1 profit (Combines Mazda, Fuji Heavy, Denso earnings)
By Yoko Kubota
TOKYO, July 31 (Reuters) - Japan's second-tier automakers Mazda Motor Corp and Fuji Heavy Industries Ltd posted double-digit profit growth in April-June helped by strong sales in the United States, their biggest market.
Mazda's U.S. sales jumped 18 percent helped by the launch of its remodelled Mazda3, while Fuji Heavy, the maker of Subaru brand cars, increased its U.S. market share in the first half of the year thanks to the popularity of its Forester compact SUV.
Mazda on Thursday said April-June operating profit rose 54.4 percent on year to 56.4 billion yen ($549.1 million), compared with a 49.2 billion yen mean forecast of 11 analysts polled by Thomson Reuters I/B/E/S.
Japan's fifth-biggest automaker by global sales volume said cost cuts and a weaker yen helped April-June profit, and kept its full-year profit forecast at 210 billion yen.
But both Mazda and Fuji Heavy saw sales drop in Japan in the first quarter partly because of a sales tax hike in April.
Mazda's Japan sales dropped 21 percent, pushed down further by slow sales of the Demio subcompact, known as Mazda2 overseas, ahead of remodelling.
Fuji Heavy, which delayed the launch of its new Levorg station wagon in Japan by about a month, saw domestic sales drop 13.5 percent, in contrast to a 6 percent rise in the United States.
For April-June, Fuji Heavy booked its highest ever first-quarter operating profit of 78.7 billion yen, up 13 percent, roughly in line with the 80.6 billion yen estimate of 11 analysts.
Its operating profit margin was 13.2 percent for the quarter, likely the highest among Japan's eight passenger car makers.
Japan's smallest passenger car maker stuck to its record-high annual operating profit outlook of 340 billion yen.
"Because we are already manufacturing vehicles at maximum capacity, there is a limit to increasing the number of vehicles we sell," Chief Financial Officer Mitsuru Takahashi told a news conference.
Earlier, Denso Corp, the world's second-biggest auto parts maker, posted a 16.7 percent drop in April-June operating profit as domestic labour costs rose and as automakers sold more compact cars in Japan that tend to be less profitable.
Shares of Mazda closed 1.3 percent higher ahead of the earnings, versus a 0.2 percent decline in the benchmark index . Fuji Heavy closed down 2.0 percent, and Denso ended down 1.8 percent. (Editing by Christopher Cushing)