The history of a mind-body problem has been aimed at the unity of mind and body. However, the unity of mind and body does not finally solve the problem. This paper is intended as an investigation of mind-body problem from the point of view of existence in an athelete. The procedure of this paper is as follows: first, the author makes the figures of a things-body and an instrumental-body clear, quoting Heidegger's ontology; secondly, the author reveals a transfiguration from those bodies to a subject-body in sport activities; and thirdly, the auther considers an existential nature of the subject-body in an athlete. It can be concluded that the subject-body leads to self-understanding, and that self-advancement bigins with the subject-body.