A frequency adjusting unit adjusts a frequency of the clock signal to be supplied to a drive unit when an image is to be formed on the second face in response to a contraction ratio of the printing material on which an image has been formed on the first face. The phase difference determining unit determines a phase difference between the clock signal corresponding to the first face and the clock signal corresponding to the second face in response to a frequency difference and a sign thereof between the clock signal corresponding to the first face and the clock signal corresponding to the second face. The change control unit changes from the clock signal corresponding to the first face to the clock signal corresponding to the second face.