In an in-vehicle illumination device including: a room lamp section provided with a first light source section, and a light guide plate that guides a light of the first light source section from an end surface thereof and emits the light from a front surface thereof; a first switch for use by the room lamp section; and a vanity mirror section provided with a mirror, when the first light source section is turned on through operation of the first switch, a mode change is made to a room lamp illumination mode in which a first illumination light comes from the room lamp section, and also when the vanity mirror section is in use, the first light source section is turned on, and a mode change is made to a vanity mirror illumination mode in which a second illumination light comes from the room lamp section. The resulting in-vehicle illumination device can be excellent in practicality with the smaller size, and can achieve improvement in terms of design.