A positive-side heat sink and a negative-side heat sink are both disposed on a generally flat plane perpendicularly intersecting a shaft, the longitudinal direction of the positive-side diodes secured to the radially inner positive-side heat sink being disposed along the radial direction of the positive-side heat sink, and the longitudinal direction of the negative-side diodes secured to the radially outer negative-side heat sink being disposed along the circumferential direction of the negative-side heat sink.