A printer prints on two sides of a sheet of paper having continuous pages, and discharges the paper to a postprocessor for disconnecting the paper in page units and storing the disconnected sheets of paper in a stack. Depending on whether the postprocessor has an inversion mechanism that inverts front and back sides of the paper, sides of the paper on which data for the front side of the paper and data for the back side of the paper are printed are switched. With this arrangement, even when the postprocessor does not have the inversion mechanism, the front side data is printed on the back side, and the back side data is printed on the front side thereby enabling the postprocessor to stack the disconnected sheets of paper in page units in the order of page numbers.