Image forming apparatus that switches transfer bias based on detected environment condition

An image forming apparatus includes an image bearing belt of a multi-layer structure, a toner image forming device, a nip forming device, a transfer power source, an environment-condition detector, and a controller. The toner image forming device forms a toner image on a surface of the image bearing belt. The transfer power source outputs a transfer bias including a superimposed voltage, in which a direct current voltage is superimposed on an alternating current voltage to transfer the toner image from the image bearing belt onto a recording medium at a transfer nip. The environment-condition detector detects an environment condition. The controller controls the transfer bias output from the transfer power source to perform a bias switching process to switch the transfer bias between a transfer bias including the superimposed voltage and a transfer bias including a direct current voltage based on a detected result of the environment-condition detector.