PURPOSE: To prevent thermal energy from radiating off a light receiving plane during night or a cloudy time by installing a light receiving plane protective cover.

CONSTITUTION: A protective cover 1 can be opened an closed around a closing center line 4 at the east side or the west side of a light receiving face and protects a light receiving plane 2 in a time band in which solar rays are not received and further, prevents thermal energy from radiating out of the light receiving plane 2. The protective cover 1 is opened and closed by means of a closing system 5, reflects solar rays when receiving solar rays to project the rays into the light receiving plane 2, thus, increasing the light receiving quantity of the light receiving plane 2. The light receiving plane 2 is installed facing to the south, inclined by a θ1 angle against a horizontal plane and inclined by a θ2 angle against the horizontal plane facing to the east.