To ensure smoothness of a sliding part of a pilot clutch and magnetic force required for coupling.
The coupling comprises a case-type torque transmission member 3 and a shaft-type torque transmission member 9, a main clutch 11 located between the torque transmission members, a pilot clutch 19, an electromagnet 51 and an armature 53 for coupling the pilot clutch 19, a rotating member 31 made of magnetic material, rotating with the case-type torque transmission member 3 and forming a part of a magnetic circuit, and a cam mechanism 13 operative to couple the clutch 11 via a pressing member 15 in response to the transmission torque of the torque transmission members 3, 9 when the pilot clutch 19 is coupled. The pilot clutch 19 comprises outer plates 75 engaging with the case-type torque transmission member 3 and inner plates 77 engaging with the cam mechanism 13, located among the outer plates 75, and sliding mutually with the outer plates 75. A smoothing groove 85 is formed in both sides of the inner plate 77.
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