A light source device includes: a light guide plate having a first internal reflection plane and a second internal reflection plane which face each other; one or more light sources each applying illumination light through a side surface of the light guide plate into an interior thereof; and an optical device disposed to face the light guide plate, and modulating, for each of partial regions thereof, a state of light rays exiting therefrom. One or both of the first and the second internal reflection planes each have scattering regions each allowing the illumination light from the light sources to be scattered and exit from the first internal reflection plane of the light guide plate, and one or both of the first and second internal reflection planes each have total-reflection regions allowing the illumination light from the light sources to be reflected in a manner of total-internal-reflection.