The present invention relates to a backlight unit (10) for an outside mirror of an automobile, comprising: a case (20) provided with an inner space for accommodating a stacked assembly (95) of components, and having a side opening (22) through which the stacked assembly (95) can be inserted; and a light source unit (30) mounted into the inner space of the case (20) and capable of emitting light when power is supplied, wherein the stacked assembly (95) comprises: a light guide plate (40) for diffusing, across the entire area thereof, the light emitted from the light source unit; and a reflection plate (50) stacked on the rear surface of the light guide plate and reflecting, to the front surface of the light guide plate, the light emitted from the rear surface of the light guide plate.