DeNA Co., the social website operator that's working with Nintendo Co. on smartphone games, and condominium builder Haseko Corp. will join Japan's Nikkei 225 Stock Average.

The companies will be added to the measure after the close on Sept. 30, replacing Nitto Boseki Co. and Heiwa Real Estate Co., index compiler Nikkei Inc. said in a statement Friday.

"The inclusion of DeNA, at least for me, is a surprise," said Satoshi Tanaka, an analyst at Daiwa Securities Co. "But it makes sense, as its agreement with Nintendo has boosted shares as well as liquidity."