PURPOSE: To reduce the number of times of division to a large extent by dividing the printing pattern data of one line and optimizing the number of times of division within a range of the max. number of drive dots capable of simultaneously driving the heating elements constituting a thermal head when the thermal head is subjected to time sharing driving according to the number of times of division.

CONSTITUTION: A black data judge/counter part 3 counts the number of ON-dots of printing pattern data and an ROM 6 stores the max. number of drive dots showing an upper limit value capable of simultaneously driving the heating elements constituting a thermal head 10. At each time when the number of ON-dots reaches the max. number of drive dots, the data up to the dot position thereof is set to one block data to divide the printing pattern data at every block unit and, according to this number of times of division, the thermal head 10 is subjected to time sharing driving at every block unit.