Each shading plate comprises a rotating shaft and a driving shaft arranged vertically. First divided shading plate is loosely fitted to and supported by (not adhere to) the rotating shaft and rotates around the shaft. Second divided shading plates are fixed to the rotating shafts and rotate by rotating movement of the shafts. A driving shaft is connected longitudinally to (not adhere to) an edge of the first divided shading plate and supply power to rotate the first divided shading plates. Recesses are formed at the center of upper short side and at the center of lower short side of the first divided shading plate and the first divided shading plate and the second divided shading plates which form one and the same shading plate are so constructed that their short sides are overlapped each other.