A flap gate comprising: a rotation bearing adapted to be provided at an opening of a structure; a substrate adapted to be provided at the bottom of the opening; a flapping door configured to flap with respect to the substrate; and a watertight elastic sheet for ensuring water tightness between the flapping door and the substrate, characterized by the rotation bearing comprising: a housing adapted to be disposed at the bottom of the opening; a door base disposed at the flapping door of the flap gate, wherein the watertight elastic sheet is disposed on a top surface of the door base; a first rotating member rotationally supported by the housing via a first shaft; a second rotating member rotationally supported by the housing via a second shaft having an axis being different from the axis of the first shaft; a synchronizing member rotationally connected to the first rotating member and the second rotating member so as to synchronize rotations of the first rotating member and the second rotating member; and connecting members (71, 72, 73) that rotationally connect the door base to the first rotating member and the second rotating member with different axes (91, 92, 93), wherein, when the flapping door is laid flat, a pivot of the flapping door is at a higher position than the axes (91, 92, 93) for rotationally connecting the connecting members (71, 72, 73) to the door base, the pivot being located near or in the watertight elastic sheet. a rotation bearing adapted to be provided at an opening of a structure; a substrate adapted to be provided at the bottom of the opening; a flapping door configured to flap with respect to the substrate; and a watertight elastic sheet for ensuring water tightness between the flapping door and the substrate, characterized by the rotation bearing comprising: a housing adapted to be disposed at the bottom of the opening; a door base disposed at the flapping door of the flap gate, wherein the watertight elastic sheet is disposed on a top surface of the door base; a first rotating member rotationally supported by the housing via a first shaft; a second rotating member rotationally supported by the housing via a second shaft having an axis being different from the axis of the first shaft; a synchronizing member rotationally connected to the first rotating member and the second rotating member so as to synchronize rotations of the first rotating member and the second rotating member; and connecting members (71, 72, 73) that rotationally connect the door base to the first rotating member and the second rotating member with different axes (91, 92, 93), wherein, when the flapping door is laid flat, a pivot of the flapping door is at a higher position than the axes (91, 92, 93) for rotationally connecting the connecting members (71, 72, 73) to the door base, the pivot being located near or in the watertight elastic sheet. a housing adapted to be disposed at the bottom of the opening; a door base disposed at the flapping door of the flap gate, wherein the watertight elastic sheet is disposed on a top surface of the door base; a first rotating member rotationally supported by the housing via a first shaft; a second rotating member rotationally supported by the housing via a second shaft having an axis being different from the axis of the first shaft; a synchronizing member rotationally connected to the first rotating member and the second rotating member so as to synchronize rotations of the first rotating member and the second rotating member; and connecting members (71, 72, 73) that rotationally connect the door base to the first rotating member and the second rotating member with different axes (91, 92, 93), wherein, when the flapping door is laid flat, a pivot of the flapping door is at a higher position than the axes (91, 92, 93) for rotationally connecting the connecting members (71, 72, 73) to the door base, the pivot being located near or in the watertight elastic sheet.