PURPOSE: To perform a high-quality printing by a method wherein when a raster type printer is not coincident with an application software in resolving power, an image is stored in a frame buffer with the resolving power of the application software, and the resolving power is converted at the time of printing.

CONSTITUTION: A clock generating means 1 consists of a quartz oscillating circuit and a dividing circuit. A linear interpolating means 2 is a two-dimensional linear interpolating means consisting of two DDAs : one takes charge of X axis, and the other takes charge of Y aixs. A resolving power conversion means conducts the address conversion between a printer 8 and a frame buffer 3 through the linear interpolating means 2 for two X and Y axes and reads a plurality of pixels. Proportion factors 4a corresponding to the pixels are simultaneously read from a table 4 to be accumulatingly multiplied, whereby a brightness 5a of the pixels to be printed by the printer 8 is found. Because the data intact cannot be directly printed by the printer 8, the pixel brightness 5a is binary coded by a threshold table 6, and the data is printed. Alternatively, the pixel brightness 5a is converted to a pulse width or pulse number by the threshold table 6, the magnitude of the pixel to be printed is converted, and the brightness is modulated.