PURPOSE: To enhance the measuring accuracy of incident angle by specifying the constitution of a read circuit.

CONSTITUTION: A read circuit to be connected to photo detectors 230 to 238 which are provided on a photo receiving plate 22 is composed of buffer amplifiers 50 to 58, adders 61 to 58, comparators 71 to 77, and a zero detector 78. When the sunlight entering the photo receiving plate 22 successively scans the photo detectors 230 to 238, the generated current passes through the buffer amplifers 50 to 58, and is fed into the adders 61 to 68. In consequence, the output of the adders 61 to 58 is fed into the comparators 71 to 77. When the output of the adder 68 which picks up the difference between outputs of photo detectors 230 and 238 is above the specified level in the zero detector circuit 78, and a read pulse RP is outputted, the outputs of the comparators 71 to 77, which are angle data, are read. Then, adjusting the photo receiving area of photo detectors 230 to 238 to the measuring angle, and matching the timing of angle reading, the angle is measured without analog error.