An AC power source voltage regulator is disclosed in which the instantaneous AC power source voltage is detected by a filter and a phase shift circuit, and the output voltage of the filter is compared by a comparator with a reference voltage. A pulse generator is actuated to give the output pulse in response to the output of the comparator and of the output of a slope regulating circuit so that in response to the output pulse a thyristor inserted in a power source circuit may be triggered.