An image forming apparatus includes a power source and a controller. A time-averaged value (Vave) of a voltage output from the power source has a polarity in a transfer direction, and an absolute value of Vave is greater than a midpoint value (Voff) of the voltage intermediate between a maximum value and a minimum value of the voltage. With an increase in a difference (Vpp) between the maximum value and the minimum value of the voltage, the controller controls the power source to increase a duty ratio (Duty) expressed by A/(A+B), where A is an application time in one cycle at a return direction side opposite a transfer direction side relative to Voff and B is an application time at the transfer direction side relative to Voff, and with a decrease in (Vpp), the controller controls the power source to reduce Duty.