PURPOSE: To surely return a reading optical system to a reading start position, to improve the efficiency of a reading operation and to miniaturize the device by deciding a scanning distance based on a detection mark provided just before the reading start.

CONSTITUTION: At the time of self-diagnosis to be performed just after a power source is turned on, a line sensor 11 performs reciprocative scanning, and a mark detection circuit 12 calculates a front edge position PF of a detection mark 4 provided on a positioning member 3 at the time of read scanning and a front edge position PR of the detection mark 4 at the time of return scanning. Based on these positions, a control circuit 13 calculates and stores a backlash amount ΔL. After read scanning is finished, a return scanning distance is decided by calculating a return scanning distance LR while using the stored backlash amount ΔL. Thus, the reading optical system can be surely returned to the reading start position, the device can be miniaturized by unnecessitating the various settings of the return scanning distance, and the efficiency of the reading operation can be improved by unnecessitating the detection of a reference mark each time of return scanning.