In an image input apparatus for compressing, in real time, image data obtained by reading an image and storing it in a memory, a memory area is ensured, an original is read and its image data is outputted, the image data is compressed in real time, the compressed image data is stored in the memory area, and it is determined whether the compressed image data is completely stored in the memory area. If it is determined that the image data is not completely stored, the compression ratio is increased, and the original read, compression of read image data, and storage of the compressed image data are repeated using the changed compression ratio.