An endoscope light source unit for making illumination light incident on a light guide, includes a rotary aperture plate, aperture openings of different opening ratios and an auxiliary-light aperture opening of a maximum opening ratio formed in the rotary aperture plate; a rotating device; an auxiliary light; a first sensing portion and a plurality of second sensing portions formed in the rotary aperture plate in association with the auxiliary-light aperture opening and the aperture openings; a first sensor which detects the first sensing portion; a second sensor which detects the first sensing portion and detects the second sensing portions; and a controller which controls the rotary aperture plate in accordance with the first and/or second sensors. When the auxiliary light is in the retracted position, the controller drives the rotary aperture plate while the second sensor detects the first sensing portion.