PURPOSE: To make a print line set accruate by improving the reliability of data obtained from a magnetic stripe, by supervising the propriety of the running speed of a printing medium by comparing a signal obtained by reading a timing mark, with a comparison signal.

CONSTITUTION: As an input signal is inputted from the sensor to input terminal 5, comparison signal generating circuits 7 and 8 generate comparison pulses, equivalent to the upper limit and lower limit allowable values of the running speed of a medium, by information signal (a) obtained by reading the timing mark given to the printing medium and they are compared with information signal (a) through AND. When the medium running speed is within the allowable speed range, logic 1 is supplied to output terminal 21; and when the running speed is above the allowable speed, logic 1 is supplied to output terminal 20 and when below the allowable speed, it is supplied to output terminals 20. Therefore, an alarm circuit, etc., are connected to output terminal 20W22 for the supervision.