PURPOSE: To reduce interpolation noise and enhance image quality of, particularly, slant lines, by inserting a low-pass filter having such interpolation characteristics as to reproduce an continuous signal which has been sampled without any interpolation noise, between a digital filter and a binary coding device.

CONSTITUTION: For pixel data inputted synchronously with a pixel clock signal for printing a pixel, a signal is outputted synchronously with a clock signal having a frequency of an integer times the frequency of the pixel clock signal. Next, an output from a digital filter 101 is smoothed by a low-pass filter 104, and an output from the filter 104 is binary coded by a binary coding device 102. The spatial spreading characteristics of a printer 103 have no spreading in the direction of continuous printing and o-th order interpolation characteristic in other directions. As a result, the printer 103 capable of continuously printing horizontally has no field of view for the pixel, and a black part is printed at an arbitrary position. Therefore, the width of a line can be made relatively uniform, and higher image quality can be contrived.