A BSM unit according to an embodiment is attached to a side surface facing the vehicle body of a visor of an outer mirror provided to a vehicle body of a vehicle. The BSM unit includes: a light source; a housing; and a lens configured to be attached to the housing and emit light from the light source to the outside of the housing. The visor has an edge portion which is positioned on a rear side of the vehicle on the side surface and is surrounding a mirror of the outer mirror, at least a portion of the lens is exposed on the side surface of the visor, and a shape of the exposure portion of the lens includes a center of the exposure portion and the edge portion and is bilateral-symmetrical with respect to a cross section orthogonal to the side surface of the visor.