PURPOSE: To maintain the distance between a photodetection surface and irradiation source blocks constant and eliminate the difference in the quantity of detected light by providing the irradiation source blocks divided to a number of pieces, elevating mechanisms for the blocks and angle varying mechanisms for the blocks.

CONSTITUTION: Radiation source blocks 3 are divided to a number of pieces and are suspended by respectively independent elevating and angle varying mechanisms 2. The mechanisms 2 are constituted of two hoisting machines for each one of the irradiation source blocks. To move an irradiation source block 7 vertically while maintaining the same horizontally, it is moved at the same rate of winding up and unwinding of the two hoists. To give an angle to the blocks 1, the rate of winding up and unwinding of one of the two hoists is increased or decreased, by which an angle is obtained. Thereby, the distance between the photodetection surface and the irradiation source blocks is maintained constant, and the difference in the quantity of detected light is eliminated.