In an image forming apparatus which has a medium thickness detecting unit for detecting a thickness of print medium and forms an image by an electrophotographic process, the apparatus has a unit for detecting the thickness of print medium that is used after a predetermined operation and a unit for detecting the thicknesses of print media on a paper feed tray at a period of a predetermined number of print pages. A target temperature of a fixing device is set on the basis of the thickness of print medium that is used after the predetermined operation and the thicknesses of print media detected at the period of the predetermined number of print pages. A waiting time is reduced and print quality can be maintained by this apparatus.