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 . <EOS>