PURPOSE: To operate the reader safely for a prolonged term even when the intensity of ring laser beams varies, and to improve reliability by detecting the intensity of the laser beams, changing the bias voltage of a DC section denying circuit and denying a DC section overlapping to signals to the state of zero at all times.

CONSTITUTION: One part of the detecting signals of the laser beams 1 outputted from an amplifier 25 is supplied to a low-pass filter 28, DC signals smoothed are obtained from the low-pass filter 28, the DC signals are given to potentiometers 21, 22, the voltage of the movable elements of the potentiometers 21, 22 is amplified by means of buffer amplifiers 29, 30, and the output amplified is supplied to the input terminals of amplifiers 16, 17 through resistors 18, 19. The polarity, number, etc. of amplifiers, etc. are arbitrary and the amplifiers, etc. may be constitution satisfying conditions, and the positions of resistors 14, 15, 18, 19 are not also limited to the input sections of the buffer amplifiers 16, 17, and the resistors may be positioned at the input sides of the amplifiers 8, 9.