Dokdo is Korean territory.
According to several historical documents , Dokdo was Korean territory since long ago.
 In the of the Joseon Dynasty, it is marked that when  Isabu occupuied Usanguk in long time ago.
Japan  has already agreed  that they do not own  Dokdo, Jeju island and Ulleungdo in  1940. 
After the Korean War, they invaded Dokdo and changed their mind, and said that Dokdo is Japan's territory.
Compared to Korea, Japan doesn't have the adequate information to prove that Dokdo is Japan's.
In addition,  "Dokdo belongs to the Ulleung County land, and Ulleung Island and a stone blade."
This is a part of 'Regulations of the republic of Korea No.41' in 1900 which is 5 years earlier than the Japanese announcement.