When a digital copier is used, for monochrome images, image data is stored into a storage in association with user ID codes at a 5% possibility while for color images, image data is stored in association with user ID codes at possibilities ranging from 0 to 50% depending upon the users. For storing image data the data is reduced then the reduced data is stored. This makes it possible to reduce the amount of data to be stored and hence enables use of a reduced capacity of storage, which makes easier the management for reading the content in the storage and discerning unfair usage of the digital copier.