When referring to the Diaoyu Islands issue, we have to mention the Treaty of Shimonoseki, which was signed in 1895 between China and Japan after the first Sino-Japanese War (1894-95).
Japan claims that the Diaoyu Islands were not included in the areas that China ceded to Japan according to the treaty items, and that the Diaoyu Islands already belonged to Japan. Therefore, article two in the treaty, which said China should cede Taiwan and its affiliated islands to Japan, didn't mention the Diaoyu Islands.
Japan believes that by saying this, it doesn't have the obligation to return China's territory. This is shady behavior.
It is widely known that the Diaoyu Islands were under China's administration since ancient times. Official documents of the Qing Dynasty (1644-1911) such as the Taiwan Prefecture Gazetteer have detailed records of administering the islands.
In the Revised Gazetteer of Fujian Province in 1871, the Diaoyu Islands were listed under the Kavalan county (now known as Ilan county), Taiwan, as a strategic coastal point.
When the Treaty of Shimonoseki was signed, the Diaoyu Islands were under the administration of Taiwan. They were ceded to Japan together with Taiwan and its affiliated islands. This is an undeniable fact.
Japan began to research the Diaoyu Islands in 1885 and seized the islands through a clandestine cabinet meeting. During the negotiation process of the Treaty of Shimonoseki, China didn't, and of course was not able to, know about such a meeting.
