When input N-bit multi-valued image information having density information is converted into M-bit (N.gtoreq.M) image information and the converted image information is output, information (S signal) indicating whether or not an image conversion process is executed for one line including a pixel of interest of the multi-valued image information is generated on the basis of the multi-valued image information. Only when the S signal indicating that the image conversion process is to be skipped successively appears for a number of times equal to or larger than "the number of lines required for the process"+1, the control is made to forcibly set the processing result of the image conversion process to be a predetermined value, and to inhibit execution of the image conversion process for one line including the pixel of interest of the multi-valued image data.