In a data filing system, a data input unit converts an input document into an electrical image signal. A data processing unit sequentially performs multi-step image compression processing including a pattern recognition step. The processing unit includes subprocessors which independently execute corresponding image compression processing of higher processing levels. A data storage unit has a hierarchical memory structure, so that, when it receives compression image data of a given image processing level from the processing unit, it can store this data in accordance with its image processing level. A system controller causes, in the document registration mode, the first subprocessor of the data processing unit to execute image compression processing of a first level, and causes the data storage unit to store the resultant compression image data. The system controller causes the remaining subprocessors to intermittently execute the subsequent image compression processing during the idle time of the filing system.