This invention provides a tool clamping/unclamping device which allows a tool to be replaced simply and securely by another one in short time, and which prevents a lock nut from being loosened, from yielding a difference in phase, and from dropping off with respect to a main shaft. The tool clamping/unclamping device comprises a tool urging means for holding a tool holding member in its clamped state, a nut positioning means for adjusting the lock nut in its phase in rotation angle position with respect to the main shaft, a nut drop-off preventing means for regulating the amount of rotation angle of the nut, a nut loosening detection means for detecting the position of the nut in the direction of the main shaft, and/or a locking mechanism for engaging a steel ball urged by a spring with a lock hole to thereby lock the lock nut in the direction of rotation. Further, for a hollow main shaft, a relay member is attached thereto, the relay member being formed similarly to the lower end portion of the solid main shaft.