In a flap valve for controlling a gas stream, having a valve tube that carries the gas stream and having a valve flap, disposed in the valve tube, that is pivotable between a closing position and an open position and is seated in a manner fixed against relative rotation on an adjustable flap shaft, to avoid openings of the shaft in the valve tube inside the sealing region between the valve flap and the valve tube, the flap shaft is oriented such that its axis forms an acute angle (&agr;) with the axis of the valve tube. The valve flap seated on the flap shaft in a manner fixed against relative rotation is oriented such that in its closing position, its surface normal is aligned with the axis of the valve tube or extends at an acute angle to it.