A control circuit for a discharge lamp, which can improve the lighting performance of the discharge lamp while realizing low power consumption, and a light source device are provided. The resistance value of the variable resistor (RC1, RC2) is not switched to the low resistance (R02) for steady discharge at the fourth time t4, but is switched to the low resistance (R02) at the fifth time t5, after lowering of power supply to the filament. With such control, a change in the lamp impedance is absorbed sufficiently by the variable resistor while power supply to the filament is being lowered and, therefore, destabilization of discharge is reduced and the lighting performance is improved.