PURPOSE: To make accurate synchronous rotation of a drive transmission shaft of a gearless shaft direction changer by providing wheels on a slide part of the gearless shaft direction changer of a robot joint part, supporting four corners by tightly applying guide parts to the wheels, and securing reciprocating motion.

CONSTITUTION: Wheels 22-27, 37-42, etc., are provided facing upper three or four surfaces and lover two surfaces of slide parts 3, 5 in a gearless shaft direction changer, and guide parts 4, 6 are tightly applied to the wheels, so the slide parts 3, 5 are supported at four corners by the three or four surfaces which make vertical reciprocating motion, where the slide part 3 is provided with support shafts 10, 11 forming a step in the perpendicular direction. A support part of the support shaft 10 and a support part of the support shaft 11 of the slide part 3 form two members to be connected to each other by a screw.