PURPOSE: To improve the accuracy of color development and to attain rapid processing by means of a simple constitution by forming conversion tables for masking processing to be applied to a color separating signal into a reference memory and executing the masking processing by using the conversion tables.

CONSTITUTION: Two processes of a process for obtaining an image signal by optically scanning an original by a reading unit A, masking the image signal in accordance with the contents of the reference memory 114 previously stored conversion table for masking and forming an electrostatic latent image on an image forming body based on a signal obtained as the result of the masking processing, and a process for developing the electrostatic latent image with color toner, are repeated to form a multicolor toner image on the image forming body. In this case, respective memory parts M1WM4 in the reference memory 114 find out color image signals Y, M, C, BK by using respective conversion tables and output the signals to a gradation correcting circuit 15. Consequently, the accuracy of color development can be improved and rapid processing can be attained by means of the simple circuit constitution.