A light source device (2, 12, 22, 24, 26) comprising: a light source (18, 27, 53) configured to emit first light in a first wavelength band; a light guide section which partial light of the first light emitted from the light source (18, 27, 53) enters, and which guides the partial light; and a wavelength conversion section (58, 171) which includes a phosphor, which other partial light of the first light emitted from the light source (18, 27, 53) enters, and which converts the other partial light into second light in a second wavelength band different from the first wavelength band, wherein the light guide section has a first end surface and a second end surface opposed to each other, and a first side surface (51c1-51c4) crossing the first end surface and the second end surface, the wavelength conversion section (58, 171) has a third end surface (58a, 171a) and a fourth end surface (58b, 171b) opposed to each other, and a second side surface (58c1-58c4; 171c1, 171c2) crossing the third end surface (58a, 171a) and the fourth end surface (58b, 171b), the first side surface (51c1) of the light guide section and the second side surface (58c1, 171c1) of the wavelength conversion section (58, 171) are disposed so as to be opposed to each other, the first side surface (51c1) is provided with a diffraction grating, the partial light enters the light guide section from the first side surface (51c1) via the diffraction grating, the other partial light enters the wavelength conversion section (58, 171) from the second side surface (58c1, 171c1), and the partial light and the second light are emitted in one of a normal direction of the first end surface of the light guide section and a normal direction of the third end surface (58a, 171a) of the wavelength conversion section (48, 171). a light source (18, 27, 53) configured to emit first light in a first wavelength band; a light guide section which partial light of the first light emitted from the light source (18, 27, 53) enters, and which guides the partial light; and a wavelength conversion section (58, 171) which includes a phosphor, which other partial light of the first light emitted from the light source (18, 27, 53) enters, and which converts the other partial light into second light in a second wavelength band different from the first wavelength band, wherein the light guide section has a first end surface and a second end surface opposed to each other, and a first side surface (51c1-51c4) crossing the first end surface and the second end surface, the wavelength conversion section (58, 171) has a third end surface (58a, 171a) and a fourth end surface (58b, 171b) opposed to each other, and a second side surface (58c1-58c4; 171c1, 171c2) crossing the third end surface (58a, 171a) and the fourth end surface (58b, 171b), the first side surface (51c1) of the light guide section and the second side surface (58c1, 171c1) of the wavelength conversion section (58, 171) are disposed so as to be opposed to each other, the first side surface (51c1) is provided with a diffraction grating, the partial light enters the light guide section from the first side surface (51c1) via the diffraction grating, the other partial light enters the wavelength conversion section (58, 171) from the second side surface (58c1, 171c1), and the partial light and the second light are emitted in one of a normal direction of the first end surface of the light guide section and a normal direction of the third end surface (58a, 171a) of the wavelength conversion section (48, 171).