PURPOSE: To prevent a temperature rise by attaching a clutch plate frictionally coupled with a clutch disc through a cushion material to a rotary field pole, and providing a space between the plate and the pole.

CONSTITUTION: A field core 11 and an armature core 4 are attached directly to a cylinder block 31. A coolant circulating passage connected to a radiator for an engine is provided at the block 31 to cool it. A clutch plate 33 is attached through a cushion material 34 having high heat resistance, made of hard rubber to a field pole 2a, and so formed as to have a space 35 in a section where the plate 33 opposes a field winding 3. The plate 33 of the section opposed to the space 36 is frictionally coupled with a clutch disc 26a. Thus, a frictional coupler generates heat, but this heat is interrupted to the pole 2a by the space 35 to suppress its temperature rise.