1. A projection exposure apparatus which projects an image of a pattern formed on a mask onto an object of exposure, comprising: a deflection detection device which detects deflection of said mask; a deflection correction device which corrects deflection of said mask; an arithmetic device which calculates a deflection correction value for said deflection correction device based on the detection result obtained by said deflection detection device; and a control device which controls said deflection correction device in accordance with the deflection correction value calculated by said arithmetic device.