A ball spline comprising: - a shaft having a plurality of axially-extending linear first spline grooves formed in an outer peripheral surface thereof; - an outer tube having the shaft passed therethrough and including second spline grooves corresponding to the first spline grooves of the shaft; and - a plurality of balls (4, 5) disposed between the shaft and the outer tube, - wherein ball circulation passages in which the balls are disposed are formed between the shaft and the outer tube by a plurality of main passages and return passages communicating with opposite end portions of the main passages, - first shoulder portions (21, 22, 23) are formed on the outer periphery of the shaft such that the first spline grooves of the shaft are interposed between the first shoulder portions (21, 22, 23) adjacent to each other in a circumferential direction of the shaft, and - second shoulder portions (24, 25, 26) are formed on the inner periphery of the outer tube such that the second spline grooves of the outer tube are interposed between the second shoulder portions (24, 25, 26) adjacent to each other in a circumferential direction of the outer tube, with the balls (4, 5) disposed in all of the main passages bear a c lockwise torque and a counter-clockwise torque transmitted between the shaft and the outer tube, - the first and second shoulder portions (21, 22, 23; 24, 25, 26) and the balls (4, 5) are arranged such that clearances are formed respectively between the first shoulder portions (21, 22, 23) and the balls (4, 5) and between the second shoulder portions (24, 25, 26) and the balls (4, 5) in a rotational direction of the shaft and the outer tube, being characterized in that - the clearances formed between the second shoulder portions (24, 25, 26) and the balls (4, 5) in the rotational direction in the return passages are made larger than the clearances formed in the main passages in the rotational direction so that the balls (4, 5) disposed in the return passages bear a torque, additionally when excessive torque is applied to the ball spline, and - the first spline grooves have a radius of curvature which is not smaller than 53% of a diameter of the balls (4, 5), and - the second spline grooves have a radius of curvature which is not smaller than 53% of the diameter of the balls (4, 5). - a shaft having a plurality of axially-extending linear first spline grooves formed in an outer peripheral surface thereof; - an outer tube having the shaft passed therethrough and including second spline grooves corresponding to the first spline grooves of the shaft; and - a plurality of balls (4, 5) disposed between the shaft and the outer tube, - wherein ball circulation passages in which the balls are disposed are formed between the shaft and the outer tube by a plurality of main passages and return passages communicating with opposite end portions of the main passages, - first shoulder portions (21, 22, 23) are formed on the outer periphery of the shaft such that the first spline grooves of the shaft are interposed between the first shoulder portions (21, 22, 23) adjacent to each other in a circumferential direction of the shaft, and - second shoulder portions (24, 25, 26) are formed on the inner periphery of the outer tube such that the second spline grooves of the outer tube are interposed between the second shoulder portions (24, 25, 26) adjacent to each other in a circumferential direction of the outer tube, with the balls (4, 5) disposed in all of the main passages bear a c lockwise torque and a counter-clockwise torque transmitted between the shaft and the outer tube, - the first and second shoulder portions (21, 22, 23; 24, 25, 26) and the balls (4, 5) are arranged such that clearances are formed respectively between the first shoulder portions (21, 22, 23) and the balls (4, 5) and between the second shoulder portions (24, 25, 26) and the balls (4, 5) in a rotational direction of the shaft and the outer tube, being characterized in that - the clearances formed between the second shoulder portions (24, 25, 26) and the balls (4, 5) in the rotational direction in the return passages are made larger than the clearances formed in the main passages in the rotational direction so that the balls (4, 5) disposed in the return passages bear a torque, additionally when excessive torque is applied to the ball spline, and - the first spline grooves have a radius of curvature which is not smaller than 53% of a diameter of the balls (4, 5), and - the second spline grooves have a radius of curvature which is not smaller than 53% of the diameter of the balls (4, 5).