To simply and accurately adjust position control of a robot arm having a winding transmission device consisting of a belt wound around two pulleys.

This tension adjusting device for the robot arm is provided with two tensioners 14 and 15 positioned to in contact with an inner surface of the belt 13 along a cross direction with respect to a line linking rotational centers of the pulleys 11A and 11B and capable of moving along a perpendicular direction, and a rod 28 having treads and perpendicularly positioned between the tensioners. The tensioner is provided with a part having threads 21 to be screwed onto the treads 20 of the rod. The threads of the tensioner and the threads of the rod are provided in a direction approaching and separating from each other by rotations of the rod.