A clutch mechanism includes a first clutch plate retaining circular columnar engagement members and a second clutch plate retaining spherical engagement members. In the second clutch plate, inner retaining holes and outer retaining holes positioned more radially outward than the inner retaining holes are formed. The outer retaining holes are larger in diameter than the inner retaining holes. On assembly of the clutch mechanism, engagement members are selected from small-diameter spherical engagement members corresponding to the small-diameter inner retaining holes and large-diameter spherical engagement members corresponding to the large-diameter inner retaining holes. The selected engagement members are disposed in the corresponding retaining holes, respectively. The maximum rotational torque is greater when the large-diameter spherical engagement members are selectively disposed than when the small-diameter spherical engagement members are selectively disposed.