In October 2002, he moved to Seoul in order to take part in the auditions held by S.M. Entertainment. he was cast in the "Starlight Casting System" and signed a contract with S.M. Entertainment and became an official trainee, being trained in areas of singing, acting, dancing, and other performing talents such as hosting and modeling. He was initially placed in a four-member group called "Four Seasons" along with Jaejoong, Yunho and Kangin. The project group was dropped when Yunho and Jaejoong joined TVXQ in early 2003. He was preparing to debut as the main vocalist of Trax, but during his trainee days he had frequent clashes and misunderstandings with the person in charge at S.M. at that time. Therefore, he became frustrated and abandoned his Trax debut and returned to his hometown in Gangwon Province. Later with Trax's Jungmo and Jay, TVXQ's Yunho and Donghae encouragement and positivity, he returned to S.M. He joined other trainees under S.M. Entertainment to form the group "Super Junior 05", the first generation of Super Junior.