The image forming apparatus according to the present invention transfers a toner image that is formed upon an image forming body by way of an image forming operation to a print paper P when the print paper (P) is conveyed toward a fixing device (8) via a transfer material (40) in a belt shape and a transfer roller (41a) having an electrode that applies a bias electric potential upon the print paper (P). The image forming apparatus includes a humidity detection sensor (11), installed upon a location close to, and is in opposition to, an obverse surface of the print paper (P), a retention roller 41c installed further downstream than the transfer roller (41a) along the direction of the conveyance of the print paper (P) and further upstream than the fixing device (8) along the direction of the conveyance of the print paper (P), which preserves the toner image transferred to the print paper (P) by applying a bias electric potential upon a surface opposite to the surface of the print paper (P) that makes contact with the transfer roller (41a), and a control device (ST) that measures the moisture content quantity of the print paper from an interval required from a commencement of a variation in humidity obtained by the humidity detection sensor (11) to a time whereat the variation reaches a maximum, and a maximum quantity of variation of the humidity, and controls the bias electric potentials according to the moisture content quantity thus measured.