An image forming apparatus includes an image forming station for forming an unfixed image on a recording material; a fixing portion for heating and fixing an unfixed image formed on the recording material, the fixing portion including a fixing rotatable member, and a pressing roller cooperative with the fixing rotatable member to form a nip for nipping and feeding the recording material; an ambient condition sensor for detecting an ambient temperature and an ambient humidity; and a controller for controlling the fixing portion; wherein the controller sets such a warming-up time for warming up the fixing portion that a temperature of the pressing roller at the time when the recording material enters the nip is a temperature calculated on the basis of the ambient temperature and the ambient humidity.