PURPOSE: To change light modulating level quickly without generating circumfluence of electric current by carrying out switching operation based on a signal from a zero cross point detecting means to apply electricity to one of a first and a second switching devices near a zero cross point and shut the other.

CONSTITUTION: Electric power to be supplied to lighting loads 2a, 2b from an a.c. electric power source 1 is controlled for light modulation at a first and a second light modulation levels respectively by a first and a second light modulating apparatus 11, 12. Switching of the first and the second light modulation levels is controlled by a method wherein first and second switching devices 21-24 connected with the first and second light modulating apparatus, respectively, are electrically conducted or shut. Since the control for switching the first and second switching devices 21-24 is able to be carried out near a zero cross point of an output electric voltage of the a. c. electric power source 1, the period in which the first and the second switching devices become in conduction state simultaneously is shortened.