Abstract: In an electromagnetic clutch having drive and driven side clutch plates in which the drive side clutch plate is provided with a plurality of elongated holes circumferentially arranged at given intervals, a depression is formed by compressing and plastically press deforming each bridge portion positioned circumferentially between adjacent two of the elongated holes on an axial end surface of the drive side clutch plate facing the driven side clutch plate. Though tensile stress caused by belt tension is likely concentrated on the bridge portion, the depression, which causes higher hardness of the bridge portion, effectively serves to restrain occurrence of the fatigue failure.