An object of this invention is to obtain a phase voltage detection circuit that can detect a phase voltage signal reliably even when a leak current flows therein and a DC bias voltage of the phase voltage signal varies. A phase voltage detection circuit that detects a phase voltage signal generated by an armature coil of an AC power generator includes a peak hold circuit that determines an envelope of the phase voltage signal using an operational amplifier, a voltage offsetting circuit that shifts an output voltage from the peak hold circuit by a preset offset amount, and a comparator that compares the phase voltage signal with an output voltage from the voltage offsetting circuit, and outputs a phase voltage detection signal.