The index had soared 118 percent since the run up to Prime Minister Shinzo Abe’s re-election in December 2012, sustained by the Bank of Japan’s (BOJ) massive quantitative easing program, which was launched in April 2013 and expanded in October 2014.

Meanwhile, the yen fell nearly 50 percent against the U.S. dollar over that period on the back of the BOJ and Abe’s efforts to spur the economy, compared to below 30 percent for the Australian dollar and just 0.32 percent for the Hong Kong dollar, which is pegged to its U.S. equivalent.

And so, despite the Nikkei’s year-to-date gains, Japan stocks still look attractive for foreign investors in dollar terms, Collett said.

In dollar terms, “the Nikkei is up 9 percent so far this year, compared to 1.4 percent for the Hang Seng and 1 percent for Australia’s ASX,” he said.

Next resistance line

The next target level will be another 15-year high: the 20,884 level last hit on April 20, 2000.

In the meantime, analysts are bullish about how high the Nikkei can climb from here, even taking into account the likelihood the U.S. Federal Reserve will raise interest rates and trigger a sell-off of stocks later this year.

“It all depends on what the Fed does, but we expect the Nikkei to hit 21,000 by the middle of the year,” said Sunrise Brokers’ Collett.

The index is likely to hit 23,000 within two years, said Collett, but in the unlikely event the current momentum is maintained for the rest of this year, the Nikkei could reach that level by the end of 2015.