A Japanese government spokesman told journalists on Tuesday that Tokyo was on top of the situation and intended to investigate.
News reports and rumour of the incident are stoking anti-Japanese sentiment in China.
Japanese Chief Cabinet Secretary Yasuo Fukuda told reporters the Foreign Ministry would question employees from a Japanese construction company.
Nine hundred took part
The incident is believed to have taken place at a five-star hotel in the southern city of Zhuhai last month.
According to Chinese media, about 400 Japanese tourists and 500 local prostitutes were involved in what sounds like the largest mass orgy of all time.
Chinese officials have detained suspects and closed the hotel in the coastal city in Guangdong Province.
The reports sparked fury in China, especially on the Internet, as the incident was said to have ended on 18 September - the 72nd anniversary of Japan's occupation of China's northeast.
Over 500 Chinese prostitutes
were allegedly involved
Fukuda said the Guangdong government had informed Tokyo on Sunday that the orgy had indeed taken place but has not provided further details as yet.
"If it took place, then it is truly regrettable," he said.
A spokesman for a construction company in western Japan - whose workers are alleged to have been most involved in the incident - acknowledged that some employees were at the hotel at the time.
However, he denied any involvement or wrong-doing.
"Our employees did not take part in any organised prostitution," the official said.
Despite closer ties in trade and tourism, and signs of growing diplomatic solidarity in the face of the North Korean nuclear crisis, antagonism towards Japan is never far below the surface in China, which suffered about 14 years of brutal Japanese occupation.