TOKYO, March 4 (Reuters) - 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)