Sets of pixel data obtained by dividing entered image data into a predetermined number of items of data are stored in storage means in a predetermined order, and conversion processing is executed for obtaining individual items of pixel data of an output image by applying predetermined processing to the stored pixel data. When the conversion processing is executed, it is determined whether a set of pixel data to be stored in the storage means next is used in conversion processing, and control is carried out in such a manner that a set of pixel data determined not to be used in conversion processing will not be stored in the storage means. As a result, processing for storing unnecessary data not used in conversion processing can be eliminated, thereby making it possible to raise the overall speed of image processing.