An image forming system includes a first data compressing device, a data decompressing device, and an image forming device. The first data compressing device stores image data of a top line to be formed on a recording medium first and image data of a designated line other than the top line in a memory area with a data format capable of forming the original image data by itself, while generating difference data which indicates a difference between image data of a non-designated line other than the top and the designated lines and the image data of a line adjacent to the non-designated line, and stores the difference data in the memory area. The data decompressing device sequentially decompresses the image data of each line, based on the image data and the difference data. The image forming device forms an image on the recording medium, based on the decompressed image data.