Patent ID: 8218986

Claim:
An image forming apparatus configured to convey a print paper toward a fixing device and to transfer a toner image formed upon an image forming body onto the print paper via a transfer roller having an electrode configured to apply a bias electric potential upon the print paper, the image forming apparatus comprising: a humidity detection sensor configured to be disposed close to and face a surface of the print paper and detect a humidity of the print paper; a retention electrode configured to be installed further downstream than the transfer roller along a direction of the conveyance of the print paper and further upstream than the fixing device along the direction of the conveyance of the print paper, and to preserve the toner image transferred to the print paper by applying a bias electric potential upon the print paper upon a surface that is upon an opposite side of the print paper to the surface of the print paper facing the transfer roller; and a control device configured to measure a moisture content quantity of the print paper based on an interval from a commencement of a variation in the humidity that is obtained by the humidity detection sensor to a maximum quantity of variation of the humidity, and control the bias electric potential of each of the electrode of the transfer roller and the retention electrode according to the moisture content quantity thus measured.