A light source device includes a light source, a coupling lens, a first opening plate, a second opening plate, a photoreceptor, a package member, a cover glass, a half mirror, and a light source control device. In relation to a main-scanning corresponding direction and a sub-scanning corresponding direction, divergence angles θm and θs of a light beam output from the light source, emission angles θm1 and θs1 of a light beam passing through an opening portion A, and emission angles θm2 and θs2 of a light beam passing through an opening portion B satisfy relationships |(θm1−θm2)/θm|≦0.085 and |(θs1−θs2)/θs|≦0.085.