A method of lighting a light source apparatus that has a discharge lamp, a reflection mirror for reflecting light emitted from the discharge lamp, a light emission optical system for irradiating a work piece with light, one or more laser oscillator for emitting a laser beam to the discharge lamp, and a discharge starting unit for starting discharge. The method includes removing deposits adhering to an inner face of the discharge lamp by irradiating a discharge vessel with the laser beam from the first laser oscillator, starting discharge in the discharge vessel by the discharge starting unit, and condensing the laser beam from a second laser oscillator, into the discharge vessel.