An image forming apparatus includes a power source (39) to output a voltage having an alternating waveform to transfer a toner image from an image bearing member (31) to a recording medium. A time-averaged value (Vave) of the voltage has a polarity in a transfer direction, in which the toner image is transferred from the image bearing member (31) to the recording medium, and an absolute value of the time-averaged value is greater than a midpoint value (Voff) of the voltage intermediate between a maximum value and a minimum value of the voltage. As a roughness of the recording medium increases, a duty ratio (Duty) expressed by A / (A+B) is reduced, where A is an area of the waveform of the voltage in a return direction opposite the transfer direction relative to the midpoint value in one cycle and B is an area in the transfer direction relative to the midpoint value.