[Object] To provide a light source device which allows the lighting starting properties of a high-pressure discharge lamp to be reliably improved without increasing the entire length of the light source device and exposing an auxiliary lamp to the outside.[Solution] A light source device 10 includes: a high-pressure discharge lamp 12; a bowl-shaped reflector 16 having an insertion hole 44 which is formed in a bottom portion 16a and in which a sealing portion 24 is inserted, an inner space 46, and a reflection surface 48 formed on an inner surface thereof; an auxiliary lamp 14 which emits ultraviolet rays UV; and a base 18 having formed therein an accommodation space 52 in which the auxiliary lamp 14 is accommodated between the base 18 and the outer side of the bottom portion 16a. The object can be achieved by setting a thickness t of the bottom portion 16a to be small such that an outer end portion 24a of the sealing portion 24 inserted in the insertion hole 44 projects into the accommodation space 52 of the base 18 and such that the entirety of an ultraviolet ray emitting space 36 of the auxiliary lamp 14 located along the side surface of the sealing portion 24 faces the sealing portion 24 in the accommodation space 52.