To reduce the number of parts, to miniaturize a device and to reduce cost, while excellently maintaining responsiveness when switching energization to an exciting coil.

This electromagnetic clutch/braking device 30 comprises an electromagnetic clutch mechanism 32 for switching transmission/disconnection of rotation of an input shaft 31 to an output shaft 33 and an electromagnetic brake mechanism 34 for braking rotation of the output shaft 33 disposed within a single case. A plate spring 50 is mounted on a hub 40 fixed to the output shaft 33. A clutch side armature 41 is mounted on a surface of an input side of the plate spring 50 and a brake side armature 42 is mounted on a surface of an output side. In the plate spring 50, a plurality of arm parts extending radially along a radial direction from a center part fixed to the hub 40 are formed, and the clutch side armature 41 and the brake side armature 42 are alternately fixed to an arm part different along a circumferential direction.