The Senkaku Islands were treated as part of the Nansei Islands of Japan under the San Francisco Peace Treaty.
After World War II, in the San Francisco Peace Treaty signed in September 1951, the Senkaku Islands were not included in those territories that Japan renounced, and instead, were placed under the administration of the United States as part of the Nansei Islands. The United States used part of the Senkaku Islands as firing ranges. At that time, China never expressed dissent. On the contrary, the Senkaku Islands continued to be treated as part of the Japanese territory by China as demonstrated by several documents including Chinese Communist Party publications and on Chinese maps at that time.
[Article on People’s Daily titled “Battle of people in the Ryukyu Islands against the U.S. occupation”, dated January 8, 1953]
[“World Atlas Collection” (1958 (reprinted in 1960))]