A light source device includes a blue laser light module that emits a blue laser light, and a dichroic mirror including an incident surface, and that reflects the incident blue laser light in a second direction different from a first direction which is an emission direction of the blue laser light so as to introduce the blue laser light into a fluorescent body, and allow part of light excited by the fluorescent body and emitted in a direction opposite to the second direction to penetrate through the dichroic mirror. The incident surface is provided with a dichroic coat region coated with a dichroic coat so as to reflect the blue laser light, and the dichroic coat region is formed on the incident surface such that a length in a direction orthogonal to the first direction is longer as a distance from the blue laser module is farther.