An mage forming device and image forming method capable of performing selective data compression. Printing data transmitted from a host computer are stored in RAM and developed into image data by CPU. The CPU determines whether or not the greatest raster length in the image data of each page exceeds a reference value that represents the data transfer ability of the laser printer (and corresponds to the value of 80% of the largest printing width of the laser printer) on a page by pages. If the greatest raster length exceeds the reference value, the image data of the page are compressed by the CPU and stored in the RAM. If the greatest raster length do not exceed the value, the image data of the page are stored in the RAM without being compressed. Then, the image data stored in the RAM are sent to an ASIC on a raster by raster basis. Only compressed image data are decompressed and the decompressed data then are transferred to a laser unit. Non-compressed image data per se are transferred to the laser unit. A printing operation is performed based on the transferred image data.