In an image forming apparatus including an image forming unit configured to transfer a developing agent to a sheet for image forming and a plurality of fixing units each configured to perform a heat-fixing process on the developing agent transferred to the sheet by the image forming unit, whether the sheet is of a type for which a plurality of fixing units are to be used for performing a heat-fixing process is determined. It is controlled so as to electrify one fixing unit of the fixing units and not to electrify the other fixing units not to be used until the number of sheets having undergone the heat-fixing process by using the one fixing unit is equal to a predetermined number if the determining determines that the sheet is of a type for which a plurality of fixing units are to be used for performing a heat-fixing process.