An image forming apparatus, comprising: a plurality of image forming units for forming toner images; a transfer unit disposed below the image forming units, wherein the transfer unit includes an intermediate transfer belt serving as an image bearing member to bear a composite toner image on a surface thereof, wherein the transfer unit includes a nip forming member to contact the surface of the image bearing member to form a transfer nip therebetween; a transfer bias output device; a controller, wherein the transfer bias output device is configured to output a transfer bias to form a transfer electric field including an alternating electric field in the transfer nip, to transfer the toner image from the image bearing member onto a recording medium interposed in the transfer nip, the transfer bias including a superimposed bias in which an alternating current bias is superimposed on a direct current bias; an information receiving device (51; 52; 50) to receive information about transfer conditions that affects transfer of the toner image from the image bearing member to the recording medium in the transfer nip, wherein the information receiving device includes a temperature detector, a humidity detector and an operation panel, wherein the transfer conditions are temperature and humidity, characterized in that the transfer conditions are further a thickness of the recording medium and a surface condition of the recording medium, wherein users can enter a type of the recording medium through the operation panel regarding the thickness and the surface condition of the recording medium, wherein the controller is configured to store a data table showing relations between temperature and humidity, and a combination of a target output value of a peak-to-peak voltage Vpp and a target output value of the offset currentIoff for each type of the recording media, wherein the controller is operatively connected to the information receiving device (51; 52; 50) and the transfer bias output device, to cause the transfer bias output device to change a target output value of a peak-to-peak voltage of the AC bias based on the information received by the information receiving device (51; 52; 50) and reduce a target output value of the DC bias as the target output value of the peak-to-peak voltage of the AC bias increases. a plurality of image forming units for forming toner images; a transfer unit disposed below the image forming units, wherein the transfer unit includes an intermediate transfer belt serving as an image bearing member to bear a composite toner image on a surface thereof, wherein the transfer unit includes a nip forming member to contact the surface of the image bearing member to form a transfer nip therebetween; a transfer bias output device; a controller, wherein the transfer bias output device is configured to output a transfer bias to form a transfer electric field including an alternating electric field in the transfer nip, to transfer the toner image from the image bearing member onto a recording medium interposed in the transfer nip, the transfer bias including a superimposed bias in which an alternating current bias is superimposed on a direct current bias; an information receiving device (51; 52; 50) to receive information about transfer conditions that affects transfer of the toner image from the image bearing member to the recording medium in the transfer nip, wherein the information receiving device includes a temperature detector, a humidity detector and an operation panel, wherein the transfer conditions are temperature and humidity, characterized in that the transfer conditions are further a thickness of the recording medium and a surface condition of the recording medium, wherein users can enter a type of the recording medium through the operation panel regarding the thickness and the surface condition of the recording medium, wherein the controller is configured to store a data table showing relations between temperature and humidity, and a combination of a target output value of a peak-to-peak voltage Vpp and a target output value of the offset currentIoff for each type of the recording media, wherein the controller is operatively connected to the information receiving device (51; 52; 50) and the transfer bias output device, to cause the transfer bias output device to change a target output value of a peak-to-peak voltage of the AC bias based on the information received by the information receiving device (51; 52; 50) and reduce a target output value of the DC bias as the target output value of the peak-to-peak voltage of the AC bias increases.