An image transfer device according to the present invention applies a transfer bias voltage to a toner image developed on a surface of an image carrying member, the transfer bias voltage having an opposite polarity from the polarity of an electric charge of the toner image, by a transfer roller from a power supply device, and electrostatically transfers the toner image to a transfer medium. A resistance element is connected between the power supply device and the transfer roller in series. The resistance element has a value of resistance which varies little with environmental variation, and a ratio between the value of resistance and a synthetic value of resistance of a transfer device including the conductive roller is set to a value which restrains variations in transfer efficiency of the electrostatic transfer with respect to variations in the synthetic value of resistance with the environmental variations.