PURPOSE: To contrive to improve the throughput of a printer and, at the same time, prevent jamming from developing when thick paper such as a block of sheets or the like is treated.

CONSTITUTION: The thickness of printing paper to deal with is detected at a step S1. Whether the detected thickness of the printing paper is thicker than a standard paper thickness or not is judged at a step S2. When the detected thickness is judged as being thicker than the standard thickness, whether the arrival of the heat travelling position at the end of the printing treatment for one line at the decelerating position of skip speed is realized or not is judged at a step S3. When no arrival is judged, the travelling speed of a printing head is set to the skip speed, which is higher than printing speed, simultaneously with the end of the printing treatment for one line at steps S5 and S6. Next, simultaneously with the arrival of the head travelling position at the decelerating position of skip speed, the travelling speed of the printing head is set to the printing speed at steps S7 and S8. In succession, simultaneously with the arrival of the head travelling position at the decelerating position of printing speed, the travelling speed of the printing head is decelerated so as to stop the printing head at the travelling starting position for the following line at steps S9 and S10.