PURPOSE: To provide a reader provided with a control circuit for automatically correcting asymmetry generated in a photoelectric circuit owing to the dirt or the like of an optical element, for example.

CONSTITUTION: An electric machine converter 20 can be moved by the low frequency vibration of smaller amplitude by a generator 37. The optical elements 27A-D transmit sum signals corresponding to correction quantity at the time of asymmetry. The sum signals pass through a band pass filter 31 whose center frequency corresponds to the center frequency of the generator 37 and are converted into correction signals by a rectifier 32. A phase detector 33 compares the vibration of the generator with the sum signal for forming a polarity signal corresponding to the polarity of the correction signal. For generating the correction signal of precise polarity, which is to be supplied to the converter 20, an operation amplifier 34 connects the output signal of the rectifier 32 to the polarity signal.