To prevent a rush current by first lighting heaters (lamps) with series connection, for preheating and attaining switching to parallel connection, after the resistance of the heaters (filaments) becomes high.

H1, H2... denote the heaters (lamps) and W denotes a switch which is linked with switches W1, W2..., to change over them. The switch W is turned upward so that a current flows through the heater H1, the contacts (a) of the switches W1 and W2, the heater H2 and an AC power source, that is, to the right side. In this state, a voltage of AC 100V is applied to a place where the heaters H1 and H2 are connected in series, so that the rush current is made low. In this state, a microcomputer keeps this state for several seconds until the heaters are warmed up. Then, the microcomputer makes the switch W a neutral state for about 0.5 second, to prevent the occurrence of an ark discharge. After that, the switch W is turned on for the side of a contact (b), to drive each heater with rated power.