In the case of delaying any one of the detected signals, which are
optically obtained from a plurality of trucks, in which the information
are respectively recorded under a state that the DC component is removed,
a zero cross waveform is extracted from a main detected signal, which is
obtained from a main truck. Then, a sub detected signal obtained from a
sub truck is held at a timing at which this zero cross waveform is
extracted. Further, a sub detected signal, which has been held during a
predetermined time frame, is averaged. Moreover, delay amounts with
respect to the sub detected signal and the main detected signal are
calculated by comparing a waveform of the averaged sub detected signal and
the zero cross waveform. Then, the present sub detected signal and the
main detected signal are delayed by the calculated delay amounts.
