A starter generator which includes a rotor having a cuplike-shaped rotor yoke and magnets attached to an inner periphery of the rotor yoke, a stator mounted on a stator bracket and a protecting cover covering said rotor and stator, in which a hermetically sealed casing is constituted by said protecting cover and stator bracket, wherein a plurality of fans are provided on an outer surface of a bottom wall portion of said rotor yoke, a plurality of vent holes are provided through said bottom wall portion, said plurality of fans constitute a centrifugal fan which draws out air inside the rotor yoke through said vent holes and sends the air to an outer periphery of the rotor yoke, and a guide surface which guides the air sent to the outer periphery of the rotor yoke to a space inside the rotor yoke is provided on the stator bracket.