The Nikkei 225 reached its peak at the end of 1989 before Japan’s asset-price crash.
The Nikkei 225 closed at a record 38,915.87 on December 29, 1989. It then fell as Japan’s extraordinary property and equity boom unwound, beginning a prolonged period often called the Lost Decades.
The bubble had been supported by rapid credit growth, soaring land prices, and strong optimism about Japanese corporations. When monetary policy tightened and asset prices turned downward, banks and businesses faced large losses and heavy debt burdens.
The Nikkei 225 is a price-weighted index of 225 prominent companies listed on the Tokyo Stock Exchange. TOPIX is a different, broader capitalization-weighted Japanese index, so it is not the precise answer to this question.