When multiple-size double-side printing in an APS mode has been selected, a system CPU first reads an original which corresponds to a second side of a paper sheet, prints data of the original on the second side of the paper sheet having the same size as the original, and stacks the paper sheet on an automatic double-side unit. Subsequently, the system CPU reads an original which corresponds to a first side of the paper sheet and prints data of the original on the first side of the paper sheet stacked on the automatic double-side unit, thus completing the double-side printing. However, where the size of an original corresponding to a second side differs from the size of an original corresponding to a first side, single-side printing is effected. The original corresponding to the first side is read and data thereof is printed on a paper sheet having the same size as the original and the sheet is discharged (single-side printing). Then, the original corresponding to the first side is read and data thereof is printed on a paper sheet having the same size as the original and the sheet is discharged (single-side printing).