According to the invention, a tandem alternator includes a rotary shaft, a first and a second power generation unit that are arranged in tandem in the axial direction of the rotary shaft, a housing, a slip ring-brush mechanism provided around a rear end portion of the rotary shaft, and a first and a second rectifier that are respectively fixed to a front and a rear end face of the housing, and a controller. The controller is electrically connected between the first rectifier and the slip ring-brush mechanism to control, at least, supply of the first field current to the first field winding. The controller is configured to form a freewheeling circuit when the first field current is interrupted. The controller is fixed to the rear end face of the housing to minimize the distance from the controller to the slip ring-brush mechanism, thereby minimizing resistance loss of the freewheeling circuit.