A rotating-tool main shaft includes a hollow ram, a hollow main shaft, a collet, a draw bar that moves the collet in the axial direction, a hydraulic cylinder that moves forwardly the draw bar, and a disc spring that moves backwardly the draw bar. The draw bar includes a spring catch which is retained in the hollow main shaft so as to be movable, and which catches the disc spring. The disc spring is placed in the hollow main shaft, and is placed within a distance that is at least a half of a distance from a tip of the draw bar to the spring catch in the axial direction.