Chinese government ships have navigated just outside of Japan's territorial waters off the Senkaku Islands for the 57th consecutive day.
Japan controls the islands in the East China Sea. China and Taiwan claim them.
57 days is the longest stretch of Chinese maritime activity since the Japanese government purchased some of the Senkaku Islands from a private Japanese owner in 2012.
Four Chinese patrol ships were spotted on Friday. The Japan Coast Guard is on the alert as Chinese vessels have encroached into the islands' territorial waters from time to time.
The Japanese government maintains the islands are an inherent part of Japan's territory, in terms of history and international law. It says there are no issues of sovereignty to be resolved over them.