A joint structure for an industrial robot is provided with a gear transmission mechanism to drive movable robot element, such as a swivel body (14), of an industrial robot. The joint structure comprises a driven gear (26) connected to the movable robot element, two drive pinions (28, 30) in mesh with the driven gear (26), respectively, at different positions on the driven gear (26), a belt transmission mechanism (28a, 30a, 32) operatively connecting the two drive pinions (28, 20), a tensioner (36) for adjusting the tension in the power transmission belt (32) of the belt transmission mechanism (28a, 30a, 32), and a rotative drive source (34) connected to one (28) of the two drive pinions (28, 30). The position of the tensioner (36) is adjusted to adjust the tension in the power transmission belt (32), to thereby eliminate backlash between the respective meshing teeth of the two drive pinions (28, 30) and the driven gear (26), by pressing the respective teeth of the drive pinions (28, 30) against the corresponding teeth of the driven gear (26), respectively, in opposite directions.