A light source device comprising: an excimer lamp that emits ultraviolet light by dielectric-barrier discharge; a flyback-type lighting device adapted to supply power to the excimer lamp, the flyback-type lighting device including: a switching element; a transformer to which the switching element is connected on the primary side and the excimer lamp is connected on the secondary side; and a control circuit adapted to supply a drive signal to the switching element, characterised in that the control circuit is adapted to control the switching element on and off such that: a switching frequency is set to a frequency FS and an ON duty is set to an ON duty TS from a time T1 at which the excimer lamp is turned on to a time T2, the switching frequency, at the time T2, is switched from the frequency FS to the frequency FO, whereby from the time T1 to a time T3, the lamp is in a starting lighting state and after the time T3, the lamp is in a steady lighting state, whereby the frequency FO is used during the steady lighting state, whereby the frequency FS for the switching element at the time T1 is lower than the frequency FO for the switching element when the lamp is in the steady lighting state, and the ON-duty TS for the switching element is increased stepwise or gradually at the time T2 to a ON-duty TO, whereby the ON-duty TO for the switching element is used during steady lighting state and the ON-duty TS from time T1 to time T3 is set to a lower value than the ON-duty TO during steady-state lighting. an excimer lamp that emits ultraviolet light by dielectric-barrier discharge; a flyback-type lighting device adapted to supply power to the excimer lamp, the flyback-type lighting device including: a switching element; a transformer to which the switching element is connected on the primary side and the excimer lamp is connected on the secondary side; and a control circuit adapted to supply a drive signal to the switching element, characterised in that the control circuit is adapted to control the switching element on and off such that: a switching frequency is set to a frequency FS and an ON duty is set to an ON duty TS from a time T1 at which the excimer lamp is turned on to a time T2, the switching frequency, at the time T2, is switched from the frequency FS to the frequency FO, a switching element; a transformer to which the switching element is connected on the primary side and the excimer lamp is connected on the secondary side; and a control circuit adapted to supply a drive signal to the switching element, characterised in that the control circuit is adapted to control the switching element on and off such that: a switching frequency is set to a frequency FS and an ON duty is set to an ON duty TS from a time T1 at which the excimer lamp is turned on to a time T2, the switching frequency, at the time T2, is switched from the frequency FS to the frequency FO, whereby from the time T1 to a time T3, the lamp is in a starting lighting state and after the time T3, the lamp is in a steady lighting state, whereby the frequency FO is used during the steady lighting state, whereby the frequency FS for the switching element at the time T1 is lower than the frequency FO for the switching element when the lamp is in the steady lighting state, and the ON-duty TS for the switching element is increased stepwise or gradually at the time T2 to a ON-duty TO, whereby the ON-duty TO for the switching element is used during steady lighting state and the ON-duty TS from time T1 to time T3 is set to a lower value than the ON-duty TO during steady-state lighting.