To provide an electromagnetic coupling device capable of restoring the constant speed rotation even if the rotating speed is high and stopping a rotor after a certain turn without causing the rotor to turn together with the rotor, and to provide an image forming device equipped with the coupling device.

The electromagnetic coupling device is structured so that the rotor 105 is fitted in a yoke 104 having a fitting part. The yoke 104 has a groove 114, and the rotor 105 has a pawl 115. A gradient part 113 is formed in the fitting part of the yoke 104 and rotor 105.