To reduce an extent of loading by balancing a loading rate between a pair of switching elements 21, 22.

A discharge lamp dimmer has an inverter circuit 200 and a chopper circuit 100. One switching element 21 of the inverter 200 is also used as a chopper switch for the chopper circuit 100. The pair of the switching elements 21, 22, which define a control property of a switching control circuit 29 belonging to the inverter circuit 200 so as to set short an on-state period T21 of the switching element 21 also used as the chopper switch and set long an on-state period T22 of the remaining switching element 22, bear the load accompanying with an inverter operation. Difference between heavy load/light load is applied by varying the duty of respective switching elements 21, 22. Then, the switching element 21 bearing the light load is borne to be also used as the chopper switch.