TOKYO (AP) _ The Tokyo stock market responded with a hearty rally today to to government efforts to bolster the financial system, boosting the Nikkei average by 4 percent to finish above 15,000 points.

The dollar slipped against the Japanese yen.

The 225-issue Nikkei Stock Average rose 617.02 points, or 4.21 percent, closing at 15,267.76. On Wednesday, the average had gained 341.33 points, or 2.38 percent, a day after plunging 4.15 percent to 14,309.41, its lowest close since March 12, 1986.

Traders attributed the gains to a sense of reassurance fostered by measures announced by the finance minister Tuesday to stabilize the market and the endorsement those steps received Wednesday from central bank governor Yasushi Mieno.

''The market feels that the government is finally serious about the falling stock market,'' said Masaru Takagi, general manager at Fuji Research Institute Corp.

Other traders said the feeling is more of relief that the market may be getting badly-needed help - and not quite optimism for the future.

''People are not so excited about the upside - they are more assured that now that we are getting support from the Ministry of Finance and the Bank of Japan that maybe we have hit bottom,'' said Tim Hayashida, an analyst with Merril Lynch.

The new measures would discourage companies from selling stocks to realize capital gains, temporarily suspend rules requiring minimum dividends from financial institutions and provide tax relief for companies burdened with bad debts.

Toshio Sumitani, an analyst with Tokyo Securities, said the market was reassured that the Finance Ministry followed up on its measures by calling on financial institutions this morning to refrain from dumping stocks in order to register capital gains ahead of half-year statements next month.

But Sumitani said buying was not very ''high-spirited'' since major players held short positions after selling heavily in the past few weeks.

Other analysts warned that Japanese financial institutions, which reportedly hold about 40 percent of all outstanding stocks in Japan, still want to lighten their portfolios and may do so after the half-year statements are out.

The broader Tokyo Stock Price Index of all issues listed on the first section rose 46.65 points, or 4.17 percent, to 1,141.44. The TOPIX rose 14.62 points, or 1.33 percent, on Wednesday.

Trading remained relatively thin with an estimated 300 million shares changing hands on the first section, up from Wednesday's 220 million shares. Advancing issues outnumbered declines 950 to 66, with 80 unchanged.

Meanwhile, the dollar closed at 126.37 yen, down 0.04 yen from Wednesday's close and also below its overnight New York finish at 126.57 yen.

After opening at 126.57 yen, it ranged from 126.15 yen to 126.63 yen. Spot transactions totaled $5.70 billion, down from Wednesday's $6.11 billion.

Yasushi Kurozumi, a dealer with Sakura Bank, said the continued rebound on the stock market helped bolster the yen.

Kurozumi, however, said investors are more susceptible to trends of high interest rates in Germany, leaving the mark in a central role among major currencies.

The yield on benchmark No. 129 10-year Japanese government bonds closed high at 4.730 percent from Wednesday's 4.700-percent finish. Their price fell to 109.27 points from 109.47.