PURPOSE: To prevent difference in sizes of printed materials when printing data, which have the different resolution but have the same size, are printed, by changing the frequency of the reference clock of writing data, and adjusting the sizes of the picture elements of the writing data.

CONSTITUTION: The frequency of a shift clock in a shift register, which transmits printing data into a laser printer, is adjusted. The size of one dot of a picture element itself is changed, and the printing data undergo variable power. For example, when a received document of 200 dpi through a GIII FAX is printed with the laser printer of 300 dpi, the frequency of a video clock is divided by 2/3. The clock, whose frequency is multiplied by 3/2, is used. The size per picture element is expanded by 3/2 times of the original size. This is equal to the case when a received document of 300 dpi is printed. Since the vertical direction cannot be adjusted in this way, the same data as the data in the line immediately before are inserted as the next line once in two lines in order to multiply the data by, e.g., 3/2 times. Thus the two lines are increased to three lines. This method and the like are used.