To obtain a better printing quality by incorporating a sub-step using double data loadings in a thermal printhead in a processing step for generating a control signal.

Printing row data is twice loaded in a printhead shift register 628 regarding each printing row by using the existing printhead shift register 628. The initial load is called a 'compensating load', and the second load is called a 'printing load'. Thus, the data are twice loaded to a serial input of the printhead 610 at each printing row or printing line. That is, the data are twice loaded at each information row or printing row to be printed on a medium. Accordingly, two heating element voltage supplying and activating cycles occur at each printing row. Therefore, a faster printing speed than a single loading method and better printing quality can be obtained by the double data loadings.