An amount of discrepancy between a theoretical paper feeding length and an actual paper feeding length is determined in advance, and a calibration value is determined based on the amount of discrepancy and stored in a ROM, for each of when a sheet of paper is fed while contacted by a sheet supply roller and when the sheet of paper is fed without contacted by the sheet supply roller. When a printing is performed on a sheet of paper, a feeding amount of the sheet of paper is calibrated based on one of the calibration values stored in the ROM.