flap gate having a door body 12 with an angle of inclination u03b8 of 90 u00b0 when raising of the door body 12 is completed the spring reaches its natural length as shown in fig6 a when the angle of inclination u03b8 of the door body 12 is 45 u00b0 and as shown in fig6 c the swinging force of the door body in the direction of rising is set to reach a minimum by means of the spring further when the angle of inclination u03b8 of the door body 12 is 0 u00b0 and 90 u00b0 the spring is compressed as shown in fig6 b and the swinging force of the door body in the direction of rising is set to reach a maximum by means of the spring as shown in fig6 c on the other hand if a tension coil spring 24 is used the spring reaches its natural length as shown in fig7 a when the angle of inclination u03b8 is 45 u00b0 and when the angle of inclination u03b8 is 0 u00b0 and 90 u00b0 the spring becomes extended as shown in fig7 b the compression coil spring 22 or the tension coil spring 24 is not limited to the linear coil spring as shown in fig6 or fig7 a spring which has non linear properties such as a taper coil spring a conical coil spring a cylindrical coil spring a barrel shaped spring or an irregular pitch coil spring may be used as shown in fig8 a combination coil spring 25 formed from.