PURPOSE: To make it difficult to receive the influence of noise and to improve adaptability for the passage configulation of a game ball.

CONSTITUTION: A groove 11, through which a game ball can pass, is formed on a cover 10 made of transparent acrylic resin, and a light-emitting diode 21 and a phototransistor 22, which are opposed to each other and between which the groove 11 is held, are placed on a substrate 20 covered with the cover 10. The light-emitting diode 21 and the phototransistor 22 are connected to a detecting circuit on the substrate 20 and when the space between them is interrupted by the game ball, a rectangular output signal is obtained from a waveform-shaping circuit, corresponding to the variation of an output signal from the phototransistor 22.