A light source device includes: a light guide plate having a first internal reflection face and a second internal reflection face, and having one or more side faces; one or more first light sources applying first illumination light into the light guide plate; and a second light source disposed to face a surface, of the light guide plate, corresponding to the second internal reflection face, the second light source externally applying second illumination light to the second internal reflection face. One or both of the first and second internal reflection faces each have scattering regions allowing the first illumination light from the first light source to be scattered and to exit from the first internal reflection face to outside the light guide plate, each of the scattering regions being provided with one or more pass-through regions each allowing the second illumination light from the second light source to pass therethrough.