An image data processing device, wherein an input image memory for temporarily storing the digital image data representing pixels stores the image data serially supplied from an upstream device in order of input, and reading and compressing of a specified quantity of the image data already stored in the input image memory are performed in parallel with storing of new image data every time the specified quantity of image data is stored in the input image memory. A digital copying machine including an image memory for storing image data from an image reader, allowing setting of an input image memory area, and provided with an image holding memory area, and being operable such that, at the start of input of the image data into the input image memory, the input image memory area is set in accordance with a maximum size of the original, that the original size is detected from the stored image data, that an unnecessary storage image area is deleted, and necessary image area data is transferred to the image holding memory area, when the original size is detected.