The six men were arrested on Hainan after they were invited by Chinese companies to conduct research. Four were then freed, while two remain in custody.
The Japanese Foreign Ministry reportedly said it would demand the release of its nationals held in China, of whom eight are in custody and seven have been indicted.
Last year, two male Japanese nationals were reportedly arrested in China also under suspicion of espionage. According to the Asahi newspaper, two men were detained separated in China's Shandong and Hainan provinces, where they also conducted a geological research on the hot springs.
Since late 2012, over 10 Japanese nationals have been detained in China on suspicion of espionage. According to media reports, in 2015, Beijing arrested four Japanese over alleged involvement in spying activities, with one of them indicted.