Japanese authorities on Monday questioned the 10 Japanese, including five local assembly members, who swam ashore on the disputed island the day before.
Criminal charges were not expected.
Japan’s chief Cabinet spokesman Osamu Fujimura called the landing “regrettable” because it was done without government approval.
Beijing has protested and the landing has generated demonstrations in several Chinese cities. The island is part of a disputed chain called Senkaku in Japanese and Diaoyu in Chinese.