Transfer device and image forming apparatus including transfer device

An image forming apparatus comprising:

• a transfer device having an intermediate transfer belt;
• a plurality of image carriers each disposed to face the intermediate transfer belt and carrying a toner image on a peripheral surface of the image carrier and moreover rotationally driven at an equal speed;
• and
• a secondary transfer roller transferring the toner image from the intermediate transfer belt to a sheet, wherein
• the transfer device includes:
• an intermediate transfer belt circumferentially and rotationally driven in one direction, the toner images being transferred from the plurality of image carriers onto a surface of the intermediate transfer belt in a superimposed manner;
• a driving roller around which the intermediate transfer belt is supported and which circumferentially and rotationally drives the intermediate transfer belt;
• an environment sensor detecting a temperature or a humidity around the intermediate transfer belt;
• a resistance detecting section detecting a change in a resistance value of the intermediate transfer belt;
• and
• a speed ratio adjusting section controlling 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.