A man looks at an electronic stock quotation board outside a brokerage in Tokyo January 6, 2015. Reuters/Issei Kato/Files

(Reuters) - Japanese stocks rose to a 15-year high on Thursday morning, helped by gains in financial and shipping companies, and as Sony Corp jumped on a well-received business plan.

The Nikkei share average gained 0.5 percent to 18,283.73 at the midday break after earlier rising to 18,322.50, its highest level since May 2000.

At midday, the value of trade on the Topix stood at 1.2 trillion yen, suggesting that daily value is heading for 3 trillion yen.

Market players said that a combination of factors have helped to underpin demand for Japanese stocks among foreign investors. These include news state-owned Japan Post Holdings Co's ditching of its conservative approach to investments with its $5.1 billion takeover of Australia's Toll Holdings, and the Nikkei breaking new ground on its dollar-based value.

"U.S. long-only (investors), who look at things on a dollar basis, say that the Nikkei is finally going into new ground," said a senior trader at a foreign brokerage. "Finally, we are broken out of a range we've been trading last year. The highest recently was $152.2, right now it's $154.2, so it's made that nice break out."

Japan Post also said on Wednesday it would review its banking unit's $1.7 trillion asset portfolio, signalling a major shift in investment strategy at one of the world's biggest institutional investors.

Sony soared as much as 4.0 percent to 3,300 yen, the highest since April 2010 after saying that it aims to boost operating profit 25-fold within three years by growing its camera sensors and PlayStation units.

Banking shares were higher, with Mitsubishi UFJ Financial Group rising 2.5 percent and Mizuho Financial Group up 2.7 percent.

Shippers were also in demand. Mitsui OSK Lines added 0.9 percent and Kawasaki Kisen gained 2.5 percent.

Some analysts noted overbought technical signals in the market suggest near-term profit-taking could be possible.

The toraku ratio, or up-down ration, rose to 136.58 as of Wednesday. A level above 120 signals an overbought market. The ratio is calculated by dividing the 25-day moving average of stocks on the Tokyo Stock Exchange's first section that gained by the 25-day average of those that fell.

The broader Topix rose 0.6 percent to 1,491.51, and

the JPX-Nikkei Index 400 gained 0.6 percent to 13,530.62.