Provided is a light source device for a collimator capable of improving an illuminance ratio in an inside of a visible light's radiation field to an outside thereof. The light source device for a collimator includes an LED with a light emitting portion that irradiates visible light and a visible light guide member. The visible light guide member may have a main body, a spacer, and a base. In the main body, a through-hole of a truncated conical shape is formed. The through-hole of a truncated cone shape forms a first opening at a first surface of the main body on a side of the LED and a second opening a second surface of the main body opposite to the first surface. The first opening may be smaller than the second opening. The conical surface of the through-hole having the truncated conical shape is formed as a mirror surface which reflects the visible light irradiated from the LED with high reflectance.