An image forming apparatus with a detachable fixing device that is capable of printing with high quality without printing wastefully. A storage unit stores identification information about a fixing device in association with an attribute of a sheet that is printable with the fixing device concerned. A control unit executes printing when the storage unit does not store an attribute of a sheet corresponding to a fixing device that is currently attached to the image forming apparatus. The control unit determines whether to execute printing according to an attribute of a sheet used in printing and an attribute of a sheet stored in the storage unit when the storage unit stores the attribute of the sheet corresponding to the fixing device that is currently attached to the image forming apparatus.