An image forming apparatus for forming an image on both of a first side and a second side which are the right side and the back side of recording material, which comprises an image carrier on the surface of which a toner image is formed, a transfer member for forming a transfer part between the transfer member and the Image carrier to transfer the toner image on the image carrier onto the recording material, a transfer voltage applying section for applying transfer voltage to the transfer member, a fixing member for heating the recording material, on which the toner image is transferred by the transfer member, passed therethrough to fix the toner image on the recording material, a reversing path for reversing the two sides of the recording material passed through the fixing member and returning the reversed recording material to the transfer part again, a timer section for measuring elapsed time since the recording material, on the first side of which a toner image is transferred, passes through the fixing member until the transfer of a toner image on the second side is started when an image is formed on the first and second sides of the recording material, a humidity detecting section for detecting humidity, and a control section for selecting transfer voltage for transferring a toner image on the second side of the recording material based upon elapsed time measured by the timer section and humidity detected by the humidity detecting section.