A method for printing both sides of a paper in a printer includes the steps of (1) providing a paper supply portion for guiding the entry of paper into a printing unit, a paper output portion for guiding the paper from the printing unit to be output from the printer, and a reversing portion for reversing the orientation of the paper having passed through the printing unit to be resupplied to the inlet of the printing unit, (2) supplying a sheet of paper to the printing unit through the paper supply portion to print a front image on a front side of the paper, (3) guiding the paper on which the front image is printed to the reversing portion to reverse the orientation of the paper, (4) resupplying the reversed paper to the inlet of the printing unit, (5) printing a rear image on the other side of the paper resupplied to the printing unit, and (6) outputting the printed paper through the paper output portion.