PURPOSE: To adjust the center of the fluminous flux of a photoelectric beam with ease by irradiating the photoelectric beam projected in a light projecting part to a photoelectric transducer and displaying the quantity of photoelectricity changing in accordance with the quantity of electricity generated in the photoelectric transducer.

CONSTITUTION: The photoelectric beam projected by a light projecting part enters the cylindrical guide 16 of a deciding device 10 for the center of the luminous flux in a photoelectric device and is irradiated to a photoelectric transducer 11. The transducer 11 converts the beam to the quantity of electricity and applies the same to a buzzer 12, thereby producing sounds from the buzzer. If the luminous flux is offcentered, the quantity of the beam projected to the transducer 11 decreases; therefore, the quantity of electricity converted with the trasducer 11 decreases and the sounds produced from the buzzer 12 diminish. In such a case, the light projecting part is moved to the position where the sounds from the buzzer 12 are largest, whereby the luminous flux is correctly aligned.