In order to suppress cost of an initial investment by a user and running cost to the user to a low extent while substantially constantly maintaining an apparent illuminance, and further, to realize resource saving, in a case of using a fluorescent lamp FHF 32 as a discharge lamp La, a lamp supply power is started to be supplied from a value lower than 45 W as rated electric power at an initial period of an operation of the discharge lamp, a light adjustment signal is changed in response to an accumulated lighting time so that the lamp supply power can be a value higher than 45 W as the rated electric power on and after a rated lifetime of the discharge lamp La, whereby the lamp supply power (output of an inverter circuit 2) is adjusted. In the fluorescent lamp FHF 32, a light/electric power ratio becomes substantially constant when the lamp supply power is 41 W to 55 W, and accordingly, a light quantity is increased without impairing light emission efficiency of the discharge lamp La, and as a result, the apparent illuminance can be maintained substantially constantly.