TOKYO - Japan's chip-related Socionext and Disco will be added to the Nikkei share average next month in a regular reshuffle of the benchmark index, the publisher of the stock average said on Monday.

Those chip stocks will replace Sumitomo Osaka Cement, Pacific Metals and Takara Holdings, the Nikkei said.

The Nikkei 225 breached the 40,000 level for the first time on Monday to a record high and posted a 19% gain so far this year. The gains were mainly fuelled by chip-related stocks, such as Advantest and Tokyo Electron.

Both Socionext and Disco outperformed the Nikkei's annual gains, rising 55% and 47%, respectively.

Online clothing retailer ZOZO will also be added to the benchmark.

(Reporting by Junko Fujita; Editing by Jacqueline Wong)