To provide a structurally simple duplex processing device that keeps a leading end of a medium in a travel direction unchanged before and after an inversion when reversing (inverting) the medium.

A flexible sheet (medium) 101 has a first leading end 101C as a leading edge in a travel direction and a first side 101A faced up. The duplex processing device comprises an upper channel 140 for receiving the sheet 101 from a processing station, a station for processing the face-up side of the sheet 101, and a reversing module 150 for inverting the sheet and returning the sheet 101 to the upper channel 140. After a reversing operation, although a second side 101B of the sheet 101 is faced up, the leading end 101C is the leading edge. A merging part merges the inverted printed sheet 101 into the input path in order to process the face-up side of the sheet 101 in the printing station.