PURPOSE: To carry out bidirectional printing at the highest speed possible with current temperature and process speedily by controlling adjustable speed of a print head based on detecting output of a temperature sensor.

CONSTITUTION: Based on detected temperature information of a print head, the highest printing speed possible with this temperature is calculated by referring to a table or by the approximation calculation. A data necessary for controlling adjustable speed of the print head is calculated from the obtained highest printing speed and a predetermined number of steps necessary for adjusting the speed. The data necessary for controlling the adjustable speed of the print head becomes a characteristic of pulse speed versus time, which is supplied to a printer 5 through a print controller 4. Since the data necessary for controlling the adjustable speed of the print head is an optimum data calculated from the temperature of the print head, it is not necessary to print on one direction or to stop in order to cool the print head, and printing can be done always on both directions, thereby enabling high speed processing.