PURPOSE: To detect the number of balls passing through a pipe with high accuracy, by opposingly arranging a long light source and a photodetector array clipping the pipe, repetitively scanning the photodetector array and counting the output signal.

CONSTITUTION: A photodetector 22 is constituted of a long light source 19 which is connected to one side of a pipe 12 via a lot of partition plates 20a and a lot of photodetectors 21 which are arranged at the other side via a lot of partition plates 20b opposingly. When a ball 15 passes through the pipe 12, a plurality of the detectors 21 are shut, while one detector 21 is shut in case of a small object 16. Thus, in scanning an output signal from each detector 21 of the detector 22 while switching one by one, a display 23 shows the recording as shown in a bent line 24. That is, a section where light is shut due to the ball 15 and the detector 21 is shut with the small object 16 shows the recording (a) and the recording (b) respectively. A counter 18 is provided to count the number of the balls 15 and the small objects 16 with distinction, allowing to indicate the number of passing objects during a specified time.