When the emitter of the thru-beam type photoelectric sensor and Panasonic CHX-SC2 face each other along a straight line and their optical axes match, the number of LEDs which light up in the level indicator, as well as, the sound tone increases. Since the sound tone can also be heard with an earphone, the sensor checker can be used in a noisy place. Since the optical axis of a thru-beam type photoelectric sensor can be checked, beam alignment is easy. This product can be used with infrared, as well as, red beam type of thru-beam type photoelectric sensors. Since the sensor checker has a compact size (W100 x H60 x D23 mm W3.937 x H2.362 x D0.906 in), it can be conveniently carried to the place of use.