Two more Japanese citizens are being held in China on suspicion of spying, bringing the total number of such cases to four, Japanese media reported.
Japan and China last month acknowledged that two Japanese had been arrested in May on suspected spying charges - one in Zhejiang province and the other near the North Korean border in Liaoning province.
Japan's Foreign Ministry on Monday declined to confirm or deny the reports.
The two earlier cases were the only ones linked to spying allegations that the Japanese government was informed of, said Kazunari Kotake, an official at the ministry's department in charge of the safety of overseas Japanese.
The government has informed Japanese visitors in China to use extra caution, Kotake said.
Japan's national broadcaster NHK said over the weekend that the cases involve a woman in her 50s, arrested in Shanghai, and a man in his 60s, arrested in Beijing, both in June.
China enacted an anti-spying law last November, in a move experts say to increase surveillance over foreigners' activities in the country.
The four people had businesses in or connections with China and frequently visited the country, according to Japanese media reports.
In 2010, four employees of a Japanese construction company were accused of filming a Chinese military site in Hebei province but were released within weeks.