An indirect illumination device for a vehicle may include an internal panel forming a diffusion portion in a surface thereof, a light guide plate faced with the diffusion portion, a light source provided in a side portion of the light guide plate to which the light guide plate may be not faced with the diffusion portion, to irradiate light to an inside of the light guide plate, and an external panel provided with guide ribs protruding from a surface of the external panel, wherein the guide ribs support a lateral side of the internal panel and the light guide plate.