An optical code reading process and a reading stability determining process are performed while suppressing an increase in the cost of a transfer system and avoiding a decrease in work efficiency. CMOS performs imaging. Processor reads an optical code contained in image data taken by CMOS and outputs a signal upon success of reading of the optical code. Further, processor calculates a first contrast at the time of teaching and a second contrast at the time of reading success, and determines reading stability based on a decrease rate of the second contrast with respect to the first contrast.