A device for attaching a tool to a machine tool is disclosed. A cylindrical hollow portion is provided in a spindle of the machine tool, and a gripper having locking elements is provided at a deep portion thereof. On the other hand, a tool is attached in advance to an installing and removing member which can be inserted into the hollow portion of the spindle. A connecting body having an engaging body is provided at the extremity of the installing and removing member. The installing and removing member to which the tool is attached in advance is inserted into the hollow portion of the spindle and the installing and removing member is rotated through a predetermined angle whereby the engaging body comes into engagement with the locking body to lock the tool to the spindle.