In a system for variable-length coding, when image data of a plurality of documents are compressed and stored in a compression memory, the image data is divided into a plurality of areas. Then, attributes are discriminated for each area, and the result of the attribute discrimination is written to an attribute memory. Attribute counters count the numbers of the discriminated attributes. A variable-length compression section compresses the code data with a compression method in response to the attribute data written to the attribute memory, and the compressed data is written to a compression memory.