To provide a tool clamping device causing no falling of a tool for opening a collet installed on the tip of a draw bar, when unclamping the tool installed on a main spindle.

When unclamping the tool by advancing the draw bar, a front end part is closed by fitting and installing a ring spring for forming a spring in a ring shape to and in a ring-shaped groove carved on the front end part outer periphery of this collet so as not to open the advanced collet. Since the collet does not always open, the tool is extracted and inserted while push-expanding the ring spring by a head part of a pull stud by a shaft directional movement of the tool gripped by a tool replacing arm. Falling of the tool is prevented by holding the head part of the pull stud by the collet energized by the spring so as to close a front end part.