An object of the present invention is to provide a reflection sheet which can improve reflection property and concealing property, and a backlight unit capable of attempting the improvement of brightness and the prevention of lack in uniformity of the brightness through decreasing loss of rays of light using said reflection sheet. Specifically, disclosed is a reflection sheet which is provided to the back face side of the backlight unit to reduce the loss of rays of light to the back face side, and which may include a substrate sheet layer made of a white synthetic resin and a highly concealing layer overlaid to the back face side of the substrate sheet layer, wherein the highly concealing layer has luminosity (L*) of equal to or greater than 95. In addition, further disclosed is a backlight unit for use in a liquid crystal display device including this reflection sheet to the back face side of an optical waveguide plate.