A vehicular interior lighting device comprising: a main body portion configured to be fixed to an inside surface, the inside surface being a cabin side surface of a roof of a vehicle; a light source accommodated in the main body portion; a light shielding cover portion constituting a housing with the main body portion and covering the light source from below; characterized in that the vehicular interior lighting device further comprises: a light guide member having a protruding portion protruding outward from the housing toward a direction along the inside surface from a gap between the main body portion and the cover portion and guiding light emitted from the light source to a cabin, when installed in a vehicle, wherein the light guide member has a diffusion portion formed on a surface of the protruding portion facing downward and diffusing light, a first light guide portion guiding light emitted from the light source to the protruding portion, and a reflecting portion reflecting light guided by the first light guide portion toward the diffusion portion, the reflecting portion is formed in the protruding portion; the first light guide portion and the protruding portion are integrally formed; and a light emitted from the light source passes through the first light guide portion, is reflected by the reflecting portion, is diffused in the diffusion portion, and is emitted to an outside from the diffusion portion. a main body portion configured to be fixed to an inside surface, the inside surface being a cabin side surface of a roof of a vehicle; a light source accommodated in the main body portion; a light shielding cover portion constituting a housing with the main body portion and covering the light source from below; characterized in that the vehicular interior lighting device further comprises: a light guide member having a protruding portion protruding outward from the housing toward a direction along the inside surface from a gap between the main body portion and the cover portion and guiding light emitted from the light source to a cabin, when installed in a vehicle, wherein the light guide member has a diffusion portion formed on a surface of the protruding portion facing downward and diffusing light, a first light guide portion guiding light emitted from the light source to the protruding portion, and a reflecting portion reflecting light guided by the first light guide portion toward the diffusion portion, the reflecting portion is formed in the protruding portion; the first light guide portion and the protruding portion are integrally formed; and a light emitted from the light source passes through the first light guide portion, is reflected by the reflecting portion, is diffused in the diffusion portion, and is emitted to an outside from the diffusion portion. a light guide member having a protruding portion protruding outward from the housing toward a direction along the inside surface from a gap between the main body portion and the cover portion and guiding light emitted from the light source to a cabin, when installed in a vehicle, wherein the light guide member has a diffusion portion formed on a surface of the protruding portion facing downward and diffusing light, a first light guide portion guiding light emitted from the light source to the protruding portion, and a reflecting portion reflecting light guided by the first light guide portion toward the diffusion portion, the reflecting portion is formed in the protruding portion; the first light guide portion and the protruding portion are integrally formed; and a light emitted from the light source passes through the first light guide portion, is reflected by the reflecting portion, is diffused in the diffusion portion, and is emitted to an outside from the diffusion portion.