A light source device includes a light source unit configured to emit light of a first wavelength band, a light guide unit, a wavelength conversion unit configured to convert the light emitted from the light source unit into light of a second wavelength band different from the first wavelength band, and a diffusion unit configured to diffuse the light emitted from the light source unit. The light guide unit includes a first portion and a second portion. The first portion is configured to guide a part of the light emitted from the light source unit to the wavelength conversion unit, to guide remainder to the diffusion unit, and to guide light from the diffusion unit to a system in a subsequent stage. The second portion is configured to guide light from the wavelength conversion unit to the system in the subsequent stage.