To print a satisfactory image output according to a kind of image without pressure of memory resources and a decrease of transfer speed due to an increase of image data capacity, a CPU of an image processing apparatus expands print data received from an external apparatus into image data, generates an attribute signal representing an attribute of the image data expanded based on the print data, stores the image data of which the data capacity has been reduced by decreasing the number of gradations and the generated attribute signal in a frame memory, and transmits the image data stored in the frame memory to an image forming apparatus according to the attribute of the image data in a state that the attribute signal has been added to the stored image data.