The present invention is a duplex printing apparatus, which is equipped with a first image forming process unit, a second image forming process unit, a fixing section, a conveyance system, and said fixing section, and a control section. In making a switch-over between printing modes (one-sided copying, both-sided copying), the control section fixes by the fixing section the unfixed toner image on the medium formed in the printing mode preceding before the switch-over and then conveys the medium by the conveyance system to a printing start position in the printing mode following after the switch-over. With this, in making a switch-over between printing modes, even when the medium is fed back, there is no possibility that the unfixed toner image formed on the medium will be disturbed. Therefore, printing quality can be maintained and printing can be performed on the medium without waste.