To absorb a shock in case of collision and to strengthen bearing power and to enlarge an allowable quantity by attaching an end effector to a tip shaft through a spring which biases in the axial direction of the tip shaft and deforms in a direction perpendicular to the above axial direction.

A spring 4c is fixedly attached to a positioning plate 4a under fixed bias force. An approach sensor 5 is arranged toward a tip shaft 3 in a shock absorber 4, and collision is detected by the turning on of an approach sensor 5 based on the approach of the tip shaft 3 caused by the collision, and emergency stop is caused by the above detection, and abnormality due to the collision is displayed on an operation box. In case where the spring 4c returns after shock absorption is carried out through the spring 4c, the accurate reproduction of the position of the spring 4c is carried out by four positioning pins.