High print quality from a thermal printer is maintained while the print speed is decreasing without producing white streaks or uneven print density by controlling the hysteresis coefficient of the thermal print head 35 based on the energizing history of the thermal print head 35 and print speed control factors used for determining print speed,which is the speed at which the paper is advanced while printing. The thermal printer, comprises a hysteresis coefficient setting unit 2 for setting a hysteresis coefficient for the print head based on the energizing history of the thermal print head 35; an energizing time calculation unit 3 for calculating the energizing time during which drive signals are to be applied to the thermal print head 35 for printing based upon the hysteresis coefficient set by the hysteresis coefficient setting unit; a printing control device 4 for generating the drive signals to be applied to the print head in response to the energizing time calculated by the energizing time calculation unit 3; a print speed determination unit 5 for determining the change in the print speed and when the print speed is decreasing; and a coefficient changing unit 6 for changing the hysteresis coefficient when a change in print speed occurs causing the print speed to decrease. Preferably the coefficient changing unit changes the hysteresis coefficient to a value greater than the hysteresis coefficient value used immediately before deceleration.