In an image forming apparatus including a fixing device that uses a pressure belt as at least one of two pressing members, a warm-up time and time for shifting to a standby state are reduced with simple control without increasing power consumption. The two pressing members are brought into a press-contact state by a contacting/separating mechanism and only heating means on the upper side is turned on. When the temperature of the upper pressing member increases to temperature near a target temperature corresponding thereto, the pressing members are driven to rotate. When it is detected by temperature detecting means on the lower side that the temperature of the lower pressing member increases to temperature near a target temperature corresponding thereto, the two pressing members are brought into a separated state by the contacting/separating mechanism.