To normally apply a power to a heating resistor without magnetically saturating a transformer.

In a mode 1, a correlation voltage VRS is energized by electricity from a zero cross point (a), and only the first and second pairs of thyristors are allowed to operate, and continuous m electricity conductive cycles is realized in a phase angle 360°×m+120°, and in a non-conductive block, the mode 1 is changed to a mode 2, and in the mode 2, the correlation voltage VRS is energized by electricity from a zero cross point b, and only the third and fourth pairs of thyristors are allowed to operate, and the continuous m electricity conductive cycles is realized in the phase angle 360°×m+120°, and positive DC voltages in the mode 1 generated in a load are offset by negative DC voltages in the mode 2.