A shaft coupling includes plates fitted on ends of input and output shafts, respectively. A plurality of guide grooves are formed in the opposed surfaces of the plates, respectively, so that each of the grooves extends perpendicular to the corresponding groove formed in the other plate. A steel ball is disposed between each pair of guide grooves of the plates at a portion where the pair of grooves cross each other. When the steel balls are pushed by the driving plate, they push the driven plate while rolling in the guide grooves, with their movements restricted by a retainer in the radial direction of the plates. Thus, large power can be smoothly transmitted between the rotary members with less frictional resistance. The offset amount can be changed easily. Between the plates, there are only the steel balls and the retainer.