The print performance of an image forming apparatus is improved by adequately estimating the behavior of a sheet receiving separation air and controlling the output of separation air. A fixing control unit 50 controls a shutter drive unit 52 defining the flow passage area on the basis of the displacement detected of the separation claw 42 to control the output of separation air. Since the sheet receiving separation air exerts a force on the separation claw 42, it is possible to estimate whether or not the behavior of the sheet is appropriate by monitoring the displacement of the separation claw 42. While different types of paper may move in different ways, the influence of separation air is dominant in the behavior of a sheet which is conveyed, and therefore the sheet can be prevented from moving in an inappropriate condition by controlling the output of separation air.