PURPOSE: To promptly further smoothly perform exchanging action of a tool, by providing the first cylindrical cam turning an arm shaft, the second cylindrical cam lifting the arm shaft and a cam plate opening and closing the clamping part of a spindle.

CONSTITUTION: The first shaft 15 is placed longitudinally facing in a front chamber 13a, and the shaft 15 pivotally mounts respectively its front end part to a front plate 11 while the rear end part to a partitioning plate 14. Further the shaft 15 mounts to its peripheral central part the first cylindrical cam 16, having a continuous groove 16a, further to the peripheral rear part a gear 17 respectively in a manner unable to relatiely rotate. While the second shaft 18, in a condition that it provides a front end part 18a penetrating through the partitioning plate 14 while a rear end part 18b penetrating through a rear plate 12, a respectively longitudinally protruded. Further the shaft 18 mounts to its peripheral surface in a rear chamber 13a a cam plate 19 and the second cylindrical cam 20, having a continuous groove 20a, successively in this order in a manner unable to relatively rotate.