On Thursday, the benchmark Nikkei 225 index rose around 2% to pass the 35,000 level.
That's its highest since February 1990.
Perhaps surprisingly, Japan's tough start to the year is one factor.
The country suffered a major earthquake on New Year's Day.
Over 200 people have so far been confirmed dead.
That all left investors in subdued mood, but also made them rethink when the Bank of Japan will finally start raising rates.
Many think any move will now be delayed.
BOJ Governor Kazuo Ueda will also be watching the data.
He has said he won't move on rates until he sees signs that wages are consistently tightening.
Yet figures out this week actually showed real wages shrinking for a 20th straight month in November.
Those numbers sent the Japanese yen lower against the dollar overnight - which also helped stocks, as the weaker currency boosts the value of overseas earnings for exporters.
Hitachi, Suzuki Motor and trading house Itochu were among the day's big gainers as a result, all up around 4% in afternoon trade.
