To make it possible to hold a door body in an immobilized state without oscillating when the raising operation has been completed. A raised state holding mechanism of a flap gate for a breakwater provided with a two-folding support rod which immobilizes and supports a door body the front end side of which is supported to freely rise or lower with a rotating shaft as a supporting point, and a stay which supports this and to which is attached a guide roller the movement of which is guided by a guide rail. Between a two-rod double-acting cylinder device and an oil tank there is a raising side oil supply pathway which supplies oil to a raising side oil chamber, a raising side oil discharge pathway which discharges oil from the raising side oil chamber, and a lowering side oil pathway which supplies/discharges oil to a lowering side oil chamber. A check valve is provided to the raising side oil supply pathway, and a shut-off valve is provided to the raising side oil discharge pathway to prevent lowering of the door body during the operation of raising the door body and after raising of the door body is completed. It becomes possible to hold a door body in an immobilized state without oscillating when the raising operation has been completed.