An image forming apparatus that is capable of normally performing second side printing on a transfer material, even if the transfer material put on standby after completion of first side printing is pulled out by a user. A transfer material having image formed on its first side is fed until it is partly exposed to the outside of a printer, and is then fed to a refeeding path in the printer and further fed to a standby position prior to image formation on the second side of the transfer material. When it is detected that the transfer material caused to stop at the standby position is pulled out by a user, a warning to stop pulling out the transfer material is given to the user.