An on-vehicle generator includes a field winding, an armature winding, a bridge circuit having MOS transistors, a rotation detecting circuit that detects a rotational state based on the phase voltage of the armature winding, an excitation current controlling circuit to control the output voltage to be a regulation voltage while rotation of the rotor is detected, two overvoltage detecting circuits each detecting overvoltage when the output voltage exceeds a threshold voltage, a bridge control circuit that ON-OFF controls the MOS transistors while the overvoltage is not detected and enables the MOS transistors to be continuously ON when detecting overvoltage, and a rotation output holding circuit that outputs a signal holding the rotational state of the rotation detecting circuit when detecting overvoltage and outputs the signal to the excitation current controlling circuit.