PURPOSE: To suppress the irregularity of the heat energies of respective resistor elements generated by the irregularity of the resistor elements constituting a thermal head by preferentially dividing the difference between the resistance values between the adjacents resistor elements into resistance rank values.

CONSTITUTION: According to the addresses designated by an address generating circuit 1, the levels of 16 kinds of resistance ranks L0, L1, L2... of respective resistor elements B1-Bn are written in the table of a resistance value memory means 2. In a gradation modulating circuit 3, a gradation signal is set to the gradation level most fitted among gradation signals k1-Kn becoming the conducting time ts at the time of a reference resistance value RS so as not to generate the irregularity of the resistance values of the respective resistor elements B1-Bn at every resistance value ranks L0-L15, for example, over 64 gradations. In a latch circuit, the gradation output signals suitable for the respective resistor elements B1-Bn are inputted to a gradation signal forming circuit 5 on the basis of the gradation signals K1-Kn modulated by the gradation modulating circuit 3.