To separate a transfer material without leaving the mark of a separation pawl on an image and to separate transfer material in the case of inserting the separation pawl between the transfer material and a transfer material carrier and separating the transfer material under the condition of discharging the transfer material carrier and the transfer material by separation bias obtained by superimposing AC voltage and DC voltage applied to a separation discharge means.

By examining the correlation between separation DC output which does not leave the pawl mark on the image and the potential of the transfer material at the time of separation, an optimum separation DC output value is obtained, so that the separation DC output to select and decide an appropriate separation DC output value is changed and controlled in accordance with an image forming condition. The data of the separation DC output corresponding to the potential of the transfer material at the time of separation is stored as a table in the memory Mo of a printer controller PC, and the controller PC selects and decides the separation DC output appropriate for image formation by referring to the memory Mo on the basis of image information on the discrimination of an original by an image reader part R, drives a separation high voltage power source SVH and supplies the selected DC voltage to a separation electrifier 5h.