To attain proper synthesis at all times independently of a data form in the case of composing a plurality of image data by converting a form of image data of an RGB and CMY data form into a prescribed form and composing the image data of the RGB and CMY data form converted into the prescribed form.

An example is shown where a white original is received as a scanner image and a black/white original is received as a controller image and images at a synthesis rate of 50% are composed. When a scanner image 71 based on the black/white original is received, an RGB→CMY conversion circuit applies logarithmic transform to the received scanner image 71 with R=G=B=100% to generate an image 73 with C=M=Y=0%. On the other hand, the controller image 72 based on the black/white original is given to the conversion circuit, where logarithmic transform is applied to the controller image 72 to generate an image 74 with C=M=Y=100%. Then an image synthesis circuit synthesizes CMY data (images 73, 74) at a rate of 50% respectively to provide an output of an image 75 with C=M=Y=50%.