Contents of a copy job including a kind of a document and a number of copy sets are specified from a user interface to set a copy job, and based on such contents a total data amount of the copy job is calculated by a CPU. Further, an available memory amount of the storing unit is detected. If the total amount of the copy job is larger than the available memory amount, the document reading operation by an image input unit is suspended. The document reading operation is resumed when the available memory amount becomes larger than the total data amount.