To provide a power source device capable of improving an allowance of the treatment of its control device against it.

A flag indicating whether a high-brightness discharge lamp 8 is in a state of rated power or not is stored in a control circuit 51. A PWM drive to an N-channel FET 12 in a chopper circuit 1 is controlled by utilizing the value of a flag. When determining whether the high-brightness discharge lamp 8 is in a state of rated power or not, it can be determined by the flag alone. Further, a set value of a duty ratio of a control pulse PLS2 in a state of rated power is stored, and it can quickly get back to the state of rated power by utilizing the set value at the state of rated power. By the above, the allowance of the treatment of the control circuit 51 against the power source device is improved, and the freedom for designing the device is also improved.