Abstract: A flap gate including a water gate door body of closed shell structure having a substantially rectangular vertical transverse cross-section with its bottom edge in the lengthwise direction being detachably supported on pivotal supports in such manner that said water gate door body can be freely erected and laid, is characterized in that the width a of said vertical transverse cross-section is selected one-sixteenths or less times as small as the distance b between said pivotal supports, and that when said water gate door body is vertically erected, a contact surface along the bottom edge of said door is brought in contact with a bank wall and also restrained by said pivotal supports so as not to separate from said bank wall.