PURPOSE: To make rush current small by applying an output start signal from a control circuit to a thyrister, enlarging a continuity angle to a certain extent at the time of the initial continuity of the thyrister and interrupting a current flow to the thyrister for the predetermined time equal to or over the turn-off time of the thyrister.

CONSTITUTION: When the first power voltage waveform becomes 0 as a thyrister Q2 is made electrically continuous with a certain large continuity angle at the start of lighting, voltage to a certain extent is generated in a capacitor C4. As a result, current flowing in the thyrister Q2 stops before the first power voltage waveform becomes O. Counting from this point, time elapsed for the voltage of the second power voltage waveform to be equal to the voltage of the capacitor C4 is made equal to or over the turn-off time of the thyrister Q2, thereby causing the thyrister Q2 to begin continuity with a trigger pulse from a control circuit at the time of applying the second power voltage waveform. According to the aforesaid system, it is possible to make small current (rush current) flowing upon the continuous condition of the thyrister Q2.