To largely reduce labor and time required in closing an existing tide gate and to prevent Tsunami wave damage.

The tide gate is divided into two sections and a lower-gate inclined plane 4-1 is provided to the lower gate 4. When closing the tide gate, the lower gate 4 is drawn to a closing position. Then, on releasing a slip-preventing safety key 2-2 which controls the upper gate 3, the upper gate 3, sliding down the upper-gate support plane 2-1 and the lower-gate inclined plane 4-1 automatically, moves to the closing position and is united with the lower gate 4, so that the tide gate is closed. Thus, the tide gate closing device enables to close the tide gate by moving the lower gate 4 and uniting the upper gate 3 sliding down on its upper surface.