A high-quality image processing apparatus capable of performing image transfer processing using a predetermined work buffer even if different decoding methods are used, without physically increasing the memory capacity. Image data in page description language is inputted into a host I/F unit from an external device such as a host computer. At step S302, a print mode of the input data is examined and a coding method is selected. Next, at step S303, a decoding method corresponding to the coding is checked, and at step S304, a band height (width) is set in correspondence with the decoding method. If the decoding method is a non-realtime decoding method, the band height is reduced to half of that in case of realtime decoding method.