Abstract: In a system of supplying electric power to an induction furnace, a required heating power is directly supplied to the induction furnace from an ac synchronous generator driven by a prime mover, such as a diesel engine, via a single-phase coil. At the same time, the voltage to be supplied to the single-phase coil is rendered continuously variable by an automatic voltage regulator of the generator to render continuously variable the heating power corresponding to the voltage to be supplied, and the frequency of the voltage to be supplied is rendered continuously variable by an automatic frequency regulator of the prime mover in accordance with predetermined mutual relationships with the voltage to be supplied. In addition, at the start of power supply, the voltage to be supplied and the frequency thereof are increased with gradients for a predetermined time duration from their predetermined minimum values to their rated values in accordance with the predetermined mutual relationships.