PURPOSE:To set a good threshold value without requiring special technique and knowledge, by allowing a line reading sensor to read the reference medium printed according to predetermined density gradation and operating the read data to calculate a printing line judge threshold value to store the same. CONSTITUTION:When a passbook is read by a line reading sensor 2, the reading signal corresponding to the printing density of each line on the passbook is outputted from the line reading sensor 2 to be inputted to a comparator 7. The threshold value data stored in the non-volatile memory of a control part 4 by the aforementioned procedure is set to a threshold value latching circuit 5 and further converted to an analogue signal through a digital/analogue converter 3 to be inputted to the comparator 7. The comparator 7 compares the reading signal outputted from the line reading sensor 2 with the analogue signal of the threshold value to generate an output signal showing which one of the signals are larger. This output signal is transmitted to the control part 4 to judge whether printing is present.
展开▼
