A tool replacing device including a case member is disclosed. In the case member, engaging balls supported between a holding member and a movable member is projected into the holding member by an extruding surface of the movable member, resiliently urged by a second spring, thereby allowing a tool to be engaged. By pressing the movable member against a resilient urging force of the second spring, the engaging balls can be moved in the movable member. Also, since there are provided shock sensors for detecting a moving distance of the holding member holding the tool, collision or the like of the tool can be detected. Even if power supply is stopped, the present tool replacing device can hold the tool.