A robot hand changer has a first joint member mounted on a robot arm, a second joint member mounted on a robot hand, and an engaging member mounted on the first joint member and angularly movable with respect to the second joint member for coupling the first and second joint members against each other. The robot hand changer also has an optical signal transmission device including sets of optical fibers disposed in confronting relation out of contact with each other for transmitting control signals across mating surfaces when the robot arm and the robot hand are coupled to each other, and photoelectric transducer circuits for converting electric signals to optical signals and vice versa. The robot hand arm further includes a detecting device for detecting dirt on the mating surface.