A phase voltage measuring device for an alternating current and a measuring method thereof for detecting a voltage from a main power supply Vcc supplied to a motor, wherein the voltage is converted to a digital signal which is multiplied by a basic pulse width modulation signal for detection of the phase voltage. A phase voltage measuring device and a measuring method have the advantage of measuring a phase voltage with a simple circuit by eliminating the problems associated with a conventional phase voltage measuring device which uses a current measuring coil and a pulse converter.