An endoscope light source device includes a light source to emit light, and a speckle reduction member to scatter the light. The reduction member has a surface including thickness variation to provide the light with an optical path difference. The device also includes a drive to move the reduction member, and a lens to condense the light passed through the reduction member onto a light guide of an endoscope. The surface of the reduction member has inclination of an average inclination angle. The average inclination angle is determined so that a light quantity loss rate of the entering light into the light guide by scattering of the reduction member and refraction of the lens has a positive value equal to or smaller than an effective allowable loss rate.