A field winding type synchronous motor comprises a stator, a rotor with field windings, a brushless exciter, and a rectification circuit rectifying an output of the brushless exciter. A first circuit in parallel with the field windings includes a discharge resistor and a first switching device with a backward diode. The discharge resistor and the first switching device are connected in series. A second switching device is provided in one of the DC lines connecting the first circuit and the rectification circuit. The first switching device is controlled by a potential obtained by resistor dividing of an induced voltage in the field windings and a connection from the potential via a diode to anode adjacent to the rectification circuit on the one of the DC lines with the second switching device thereon. The second switching device is closed in a synchronous speed.