The present invention relates to a light source apparatus with a structure that improves the positioning precision of a gas discharge tube and facilitates gas discharge tube maintenance work. The light source apparatus has a lamp container housing the gas discharge tube, and an insulating socket member that is fixed in a state of being positioned with respect to the lamp container and that is housed in the lamp container together with the gas discharge tube. The insulating socket member has a plane contacting portion put in plane contact with a stem portion of the gas discharge tube, and a pin socket member into which at least one of stem pins, held in the stem portion is inserted in an electrically contacting state. By the corresponding stem pin being inserted in the pin socket member, the gas discharge tube is fixed in a manner enabling attachment and detachment with respect to the insulating socket member, and the precision of positioning of the gas discharge tube with respect to the lamp container is thereby improved dramatically. Also, because the insulating socket member fixes the gas discharge tube in a detachable state, detachment of the gas discharge tube is facilitated and consequently, exchange of the gas discharge tube and other maintenance work are facilitated.