To print a seal with a uniform image density on a part of a medium accelerated or decelerated to a printhead during printing by incorporating a step of controlling a pulse width of a strobo signal in a processing step of generating a control signal.

Tables of values of a basic pulse width at an instantaneous medium speed to a printhead 84 and values of head temperature gain constant are generated, and the respective values are set so that the printer can deal with a constant speed to be maintained by the printer. These tables are generated by using a floating point calculation from when a power is first supplied to the printer to when a printing process is started. At the time of starting a printing operation, trial printing is conducted, and a printing quality is monitored during the trial printing. The pulse width and the gain constant are altered during printing until the monitored quality becomes allowable. During actual printing, a pulse width of the strobo signal is controlled based on the tables. Thus, the quality in accelerating and decelerating areas can be improved.