The present invention provides an automotive electric motor-generator that can achieve sufficient cooling of a radiating plate by ensuring a sufficient cooling airflow ventilation channel within limited axial or radial dimensions. In the present invention, first and second radiating plates each form a fan shape, have N-channel power MOSFETs mounted thereto, and have a drain potential for the power MOSFETs. A first circuit board includes insert conductors that connect the power MOSFETs in series, and a second circuit board has insert conductors that are connected to source terminals of the power MOSFETs and that have negative potential. The first radiating plate, the second radiating plate, the first circuit board, and the second circuit board are disposed in a fan shape that is centered around a shaft so as to line up radially in a plane that is perpendicular to the shaft outside one axial end of a rear housing.