A transfer device includes an intermediate transfer belt, a driving roller, an environment sensor, a resistance detecting sensor, and a speed ratio adjusting section. The toner images are transferred from the plurality of image carriers onto a surface of the intermediate transfer belt in a superimposed manner. The driving roller circumferentially and rotationally drives the intermediate transfer belt. The environment sensor detects a temperature or a humidity around the intermediate transfer belt. The resistance detecting section detects a change in a resistance value of the intermediate transfer belt. The speed ratio adjusting section controls a rotation speed of the driving roller according to results of detection by the environment sensor and the resistance detecting section, thereby adjusting a speed ratio Vb/Vd between a circumferential rotating speed Vb of the intermediate transfer belt and a rotation speed Vd of the image carrier.