Abstract: A flap valve, particularly used for a conductor of large cross section, includes a valve housing and a flap member mounted in the housing for pivoting movement about a first axis through an angle of approximately 90.degree. between two extreme positions. A drive shaft is mounted in the housing for rotation about a second axis, and a tension lever is fixed to the drive shaft and rotatable therewith about the second axis. A joint rod is pivoted at a first end thereof to the tension lever and at a second end thereof to the flap member at a position substantially centrally thereof. Rotation of the drive shaft and the tension lever about the second axis moves the joint rod and thereby pivots the flap member about the first axis from one extreme position to the other. The second axis is positioned relative to the first axis to ensure that an angle of rotation of the tension lever to move the flap member between the two extreme positions thereof is approximately 90.degree. to 150.degree..