To easily determine in a short time whether an operating condition is good or bad by a method wherein a first counter is provided for counting the operation frequency of a reed switch and a second counter is provided for counting the rotation of a disk when the disk is rotated, and a count value of the first counter is compared with that of the second counter.

When an infrared ray beam 19 emitted from an light-emitting device is reflected from a reflection plate 17 and is interrupted by a light- interrupting device, a second counter 18 counts the light-interruption frequency and then calculates the rotation of a disk 14. With a testing proximity switch 2 fixed so as to stride over the disk 14, a terminal of the testing proximity switch 2 is connected to a first counter for counting the frequency with which the proximity switch 2 crosses a shielding plate 16. If read values of the first counter is equal to that of the second counter, the proximity switch 2 is determined to be normal. If the read values of the first counter are different from that of the second counter, the proximity switch 2 is determined to be faulty. This can accurately determine in a short time whether the proximity switch 2 is normal or faulty.