TOKYO (AFP) - A resurgent Japanese stock market has seen a big boom in shares of the country's corporate titans, but now rich returns are waiting in small firms virtually unknown overseas, according to analysts.

The likes of Sony and Toyota have seen huge gains since the start of the year as waves of overseas cash flooded into the long-lacklustre Tokyo market, nearly doubling the electronics giant's stock while the world's largest automaker jumped about 55 percent.

The Nikkei 225 stock average - a who's who of corporate Japan - has surged 36 percent since January, staging a sharp recovery as Tokyo's policy blitz of government spending and monetary easing helped set off a sharp decline in the yen.

That has boosted profitability among Japanese exporters and investors are hoping for more as earnings season gets under way this week.

But the multi-month surge on the Nikkei since late last year has stoked fears about overheating even as the index came off highs that saw it up about 80 percent at one stage.

Now, some are looking beyond members of the blue-chip index to the hundreds of other firms that populate the Tokyo Stock Exchange (TSE), one of the world's biggest equity markets.

"At first, monetary easing helped boost prices of the biggest capitalisation shares... which made small-and mid-cap stocks relatively undervalued," said Shun Maruyama, chief Japan equity strategist with BNP Paribas Securities in Tokyo.

"Foreign investors see these smaller firms as a gold mine."

In fact, some have already made it onto investors' radar screens.

Discount retailer Don Quijote's shares have doubled since January, SymBio Pharmaceuticals logged a 67 percent rally while Pickles Corp., Japan's biggest pickle maker, has soared about 85 percent since the start of the year.

The weak yen makes exporters more competitive overseas and inflates repatriated foreign profits, but they are at the mercy of the ups and downs of currency markets - smaller firms that concentrate on the domestic market are often less affected by those fluctuations.

And they are set to benefit most from Japanese Prime Minister Shinzo Abe's bid to reboot the economy by firing up consumer demand at home, analysts said.

Infrastructure firms are also set to notch up more business from disaster reconstruction projects and Tokyo's winning bid to host the 2020 Olympics.

Among the firms asset managers are buying include Sho-Bond Holdings, which repairs and reinforces concrete structures such as roads and bridges, seatbelt and airbag maker Takata, and MonotaRO Co., which sells tools and parts to smaller manufacturers.

"If you really want to find growing companies, you should target mid- and small- cap firms," said Hideo Shiozumi, a fund manager with investment giant Legg Mason and also head of Shiozumi Asset Management.

The long-time portfolio manager points to nursing care businesses that are capitalising on a huge demographic shift in rapidly ageing Japan. People over 65 make up about one quarter of the country's 128 million people and that ratio is expected to balloon due to low birth rates.

"Who knows, the future of Japan Inc. could be represented by small firms," Shiozumi said.

After Japan's late-eighties peak gave way to a deflated asset bubble and two decades of laggard growth, many foreign investors ditched small firms as they slashed their holdings in the country, making it a virtual investing pariah.

"As a result, the number of analysts watching small companies declined sharply," said Masahiro Fukuda, investment director of FIL Investment Japan, a Japanese unit of Fidelity Worldwide Investment.

That long-term trend has reversed with foreign investors pouring about US$98 billion (S$121 billion) into the Tokyo market between January and September, about three or more times buying levels last year and in 2011, according to TSE figures.

Still, the Nikkei has so far outpaced gains among more than 1,000 small and mid-sized firms on the Topix index and any future boom among the smaller players won't last forever, Fukuda said.

"The most important thing is for companies, whatever size, to take advantage of the current upbeat business environment and turn it into sustainable growth over the next few years," he said.

"The real test for investors and Japanese firms lies ahead."