A flat lightweight member that is favorable in appearance quality and also excellent in productivity while being excellent in mechanical property at the ends and adhesiveness between a core and a skin includes: skin layers disposed on both surfaces of the flat lightweight member; end reinforcing layers disposed to have contact with both inner surfaces of the skin layers on the both surfaces at the ends of the flat lightweight member; and a core layer disposed in the space surrounded by the skin layers and the end reinforcing layers to have contact with the inner surfaces of the skin layers.