PURPOSE: To allow no deterioration of printing quality even in a graphic pattern of high printing density by providing a means for thinning each data bit string which directly crosses a digit direction by a prearranged bit interval and transmitting data bits to a printing head driver and a means for transmitting the thinned-off printing data to a printing head driver.

CONSTITUTION: While the first single line data is read, an instruction is input so that only odd-numbered bits of each data bit string which directly crosses the digit direction of the first single printing data be selected and transmitted to an odd-numbered bit data buffer 3. In addition, only even-numbered bits of each data bit string which directly crosses the digit direction of the second line printing data are selected and transferred to an even-numbered bit data buffer 4. In this way, a total of two printing actions enable a single line of printing to be performed. Therefore, excess heat accumulation in a printing head can be prevented even when a high density graphic pattern is printed.