An optical fiber light source apparatus, includes a casing, a discharge lamp accommodated inside the casing, a concave reflection mirror, which holds the discharge lamp, and an optical fiber, which receives light from the reflection mirror. The concave reflection mirror, to which the discharge lamp is fixed, is held by a holding member, which has an approximately rectangular flange portion in a front face thereof and a regulating concave portion formed in a side of the flange portion. A guide is provided on the casing in a direction perpendicular to an optical axis, so that the flange portion of the holding member is slidably engaged with the guide. A pressing member, which is pressed with a bias force toward the regulating concave portion of the flange and engages with the regulating concave portion, is provided on the guide, thereby securing the concave reflection mirror.