The Japanese government says China placed a buoy off Japan's southernmost island in June. The area is considered open waters but is within Japan's continental shelf.
Chief Cabinet Secretary Hayashi Yoshimasa said on Friday that it is regrettable that China placed the buoy without explaining its purpose, plan or other details.
The government says a Chinese maritime research ship installed the buoy last month in an area in the Pacific Ocean, south of the western main island of Shikoku and off the southernmost island of Okinotorishima.
It says Japan had tracked the Chinese vessel's movements, and asked the Chinese side not to infringe on Japan's maritime interests within the continental shelf, even in the open seas.
The government says it also asked China to explain the purpose and other details of its activities, but the buoy was installed without any explanation.
Hayashi said Japan has asked Beijing to be transparent, both in its explanation and activities, given that various concerns and suspicions have been raised over China's maritime activities.
He said the Chinese side responded that the buoy was for tsunami observation, and that it does not infringe on Japan's sovereign rights over the continental shelf.
Hayashi said the government will continue to gather and analyze relevant information.