An image forming apparatus including a fixing roller; an endless fixing belt rotated together with the fixing roller; a pressurizing section arranged on the inner peripheral surface side of the fixing belt wherein the pressurizing section makes the fixing belt to be in pressure contact with the fixing roller, and corresponding to the outer peripheral surface of the fixing roller, the pressurizing section is deformed into a concave shape; a separation section which makes the fixing belt to be in pressure contact with the fixing roller, and by a tip of the separation section, the outer peripheral surface of the fixing roller is deformed into a concave shape; and a controller for controlling the pressing force of the separation section so that when an envelope is passed through as the recording material the pressing force becomes stronger than the pressing force when a plain paper passes through as the recording material.