PURPOSE: To improve the reading capability by providing a means to change the black-detecting level sensitivity corresponding to the number of pieces of thick bars in a read data.

CONSTITUTION: A central information processing circuit 11 sets the threshold level of black-detecting level sensitivity at C, and inputs its own outputs P0WP3 to a data comparator 10. If an image sensor 5 reads a bar code 2, a control circuit decides if the bar code 2 is accurately read, and in case of inaccurate reading, decides if the number of thick bars is more or less than two. And, in case the number of the thick bars exceeds two, the threshold is reset to B, but if less than two, reset to A, and the image sensor 5 is allowed to read the bar code 2 once again.