Recently the NHK News reported the following:
One year had passed since the China fishing boat collision incident which occurred in the offing of the Senkaku Islands in Okinawa Prefecture. About the Senkaku Islands, some 40 years ago while was still under the American rule, there were some opinion that opposed to their returning to Japan; they were made known within the American government, the circumstances in which that these islands were finally returned as confirmed by President Nixon based on his personal decision had been made clear in the U.S. diplomatic documents.
The Senkaku Islands came under the U.S. rule as a part of Okinawa after World War II, and the administrative power was returned to Japan under the Restoration of Okinawa in 1972. Before the returning, Taiwan, having diplomatic relationship with the United States, strongly opposed to its returning to Japan, and had worked on influencing the American government of the day. Also added to the worry of Taiwan, it was the time when the fiber negotiation was a pending question between Japan and the US; this issue was used to demand concession from Japan within the American government, and there was opinion that the islands should not be immediately returned to Japan. In face of that, the ambassador to Japan then the Under Secretary of State Johnson who was known as a pro-Japanese was found to have strongly asserted the returning to Japan, as shown in the confidential documents in the United States. Mr. Johnson, based on a chronology summary, showed that "Taiwan has at one time admitted that the Senkaku Islands were part of Okinawa", and he held the view that "Taiwan is strengthening the opinion over their sovereignty for reasons of the natural resources there". Based on this chronology, the president of the day President Nixon had discussions with Kissinger the National Security Adviser, and the result was that the President himself decided to return the Islands to Japan. About this, the Japan-U.S. relationship specialist Mr. Mikio Haruna, a specially-appointed professor of the Nagoya University, upon investigating the diplomatic documents said "This episode shows clearly that the United States is deeply involved in the problems involving the Senkaku Islands".
So, it is the American who had decided to return the Senkaku Islands to Japan in the 1970s.