PROBLEM TO BE SOLVED: To obtain an irradiation light having sufficient and uniform illumination distribution in a light source device which emits light irradiating a part to be observed.SOLUTION: A light source 52 is disposed so that a light axis of an irradiation light is made incident at a tilt with respect to a light axis of a light guide 60 which guides and emits the entered light while repeatedly performing side surface reflection of an incident light. A ramp filter 55 is provided so that transmittance of the irradiation light decreases toward the center of the light axis of the light guide 60 between the light source 52 and an incident end face of the irradiation light of the light guide 60.
