A light source device includes: a light-emitting tube including a light-emitting portion; a reflector including a neck portion provided with an insertion hole to which the light-emitting tube is inserted, and a reflecting portion integrally formed with the neck portion and having an ellipsoidal curved reflecting surface. The light-emitting tube has a sub-reflection mirror that covers substantially front half of the light-emitting portion. The reflector has a step formed between the peripheral edge of an opening end of the insertion hole near the reflecting surface. The external diameter of the step is larger than the external diameter of the sub-reflection mirror while the external diameter of the step is within the internal diameter of a valid reflection area of the reflector, the internal diameter being defined by a focal position on the front side of the reflector and the outer periphery of the sub-reflection mirror.