A rotor position judging unit detects a rotor position by utilizing induced voltages generated at respective phases of an armature winding. While a field current flowing through a field winding rises toward a target value (while the field current temporally changes), induced voltages are generated at the respective phases of the armature winding by temporal differentiation of a magnetic flux which interlinks with the armature winding. The rotor position judging unit detects the rotor position (d-axis of a rotor) on a basis of a table containing combination of amplitude ratios and polarities of the induced voltages generated at the respective phases.