The Tokyo Stock Exchange's benchmark Nikkei index on Friday briefly broke through the psychologically important 20,000 level last seen 15 years ago.
The Nikkei 225 index reached 20,006.00 in the first few minutes of trade, rising 0.34 percent from Thursday following gains on Wall Street and a stronger dollar on an encouraging US jobs report.
It was the first time that the Nikkei had hit 20,000 since April 2000.
The index later came off its high, to sit at 19,917.62, a 0.10 percent loss from Thursday.
The early rise was heavily influenced by heavy-weighted Fast Retailing, the operator of fast-fashion brand Uniqlo, whose shares were up 2.62 precent after the group announced strong half-year earnings.
Strategists are upbeat over the outlook of stock prices on expectations of good earnings at Japanese companies, while acknowledging a short-term overshoot.
"When we look back on this moment, the 20,000 figure should just be a stop along the road," said Juichi Wako, a senior strategist at Nomura Holdings.
"It wouldn't be surprising if better-than-expected earnings lift the Nikkei to 21,000 or 22,000 by year's end.
"However, above the 20,000 mark, we might begin to hear voices that stocks are a bit overvalued," he told Bloomberg News.
The dollar held steady early Friday after rising on an encouraging US unemployment claims report.
The greenback was at 120.58 yen on Friday, hardly changed from 120.59 yen in New York late Thursday but up from 120.28 yen in Tokyo earlier Thursday.
The US Labor Department reported Thursday that the number of first-time unemployment claims rose last week, but claims filed in the past four weeks fell to a nearly 15-year low, suggesting a stronger labour market.
The robust data stoked speculation about the Federal Reserve's plan to raise ultra-low interest rates, boosting the dollar in a positive move for Japanese exporters.
The euro bought $1.0661 and 128.56 yen on Friday against $1.0659 and 128.55 yen in US trade.
On Thursday big gains in petroleum stocks lifted the US equity market as investors bet on more large energy mergers following Royal Dutch Shell's huge takeover of BG Group.
The Dow Jones Industrial Average rose 0.31 percent while the broad-based S&P 500 gained 0.45 percent.