A hydraulic circuit for a ram cylinder, the hydraulic circuit comprising: the ram cylinder which drives a driven device such as a flap gate and a spillway gate; a hydraulic unit including (i) a supply circuit including a supply control valve which is connectable to a discharging side of a hydraulic power supplier and controls supply of hydraulic oil discharged from the hydraulic power supplier, and a supply throttle valve which controls an amount of hydraulic oil supplied to the ram cylinder, and (ii) a discharge circuit including a discharge control valve which is connectable to an oil tank and controls discharge of hydraulic oil returned from the ram cylinder, and a discharge throttle valve which controls an amount of hydraulic oil returned from the ram cylinder; a first supply/discharge circuit which connects the ram cylinder with the supply circuit of the hydraulic unit and a second supply/discharge circuit which connects the ram cylinder with the discharge circuit of the hydraulic unit; the hydraulic circuit being characterized in that it further comprises: a bypass circuit which connects (i) a portion of the supply circuit of the hydraulic unit, said portion being downstream from the supply control valve and the supply throttle valve and (ii) a portion of the discharge circuit, said portion being downstream from the discharge control valve and the discharge throttle valve, the bypass circuit including a stop valve having an opening/closing function, a flushing circuit for flushing the ram cylinder, the flushing circuit being established when the bypass circuit closes communication between the supply circuit of the hydraulic unit and the discharge circuit of the hydraulic unit and when the supply control valve and the discharge control valve are controlled to be in the open positions; and a driving circuit for driving the ram cylinder, the driving circuit being established when the bypass circuit establishes communication between the supply circuit of the hydraulic unit and the discharge circuit of the hydraulic unit. the ram cylinder which drives a driven device such as a flap gate and a spillway gate; a hydraulic unit including (i) a supply circuit including a supply control valve which is connectable to a discharging side of a hydraulic power supplier and controls supply of hydraulic oil discharged from the hydraulic power supplier, and a supply throttle valve which controls an amount of hydraulic oil supplied to the ram cylinder, and (ii) a discharge circuit including a discharge control valve which is connectable to an oil tank and controls discharge of hydraulic oil returned from the ram cylinder, and a discharge throttle valve which controls an amount of hydraulic oil returned from the ram cylinder; a first supply/discharge circuit which connects the ram cylinder with the supply circuit of the hydraulic unit and a second supply/discharge circuit which connects the ram cylinder with the discharge circuit of the hydraulic unit; the hydraulic circuit being characterized in that it further comprises: a bypass circuit which connects (i) a portion of the supply circuit of the hydraulic unit, said portion being downstream from the supply control valve and the supply throttle valve and (ii) a portion of the discharge circuit, said portion being downstream from the discharge control valve and the discharge throttle valve, the bypass circuit including a stop valve having an opening/closing function, a flushing circuit for flushing the ram cylinder, the flushing circuit being established when the bypass circuit closes communication between the supply circuit of the hydraulic unit and the discharge circuit of the hydraulic unit and when the supply control valve and the discharge control valve are controlled to be in the open positions; and a driving circuit for driving the ram cylinder, the driving circuit being established when the bypass circuit establishes communication between the supply circuit of the hydraulic unit and the discharge circuit of the hydraulic unit. a bypass circuit which connects (i) a portion of the supply circuit of the hydraulic unit, said portion being downstream from the supply control valve and the supply throttle valve and (ii) a portion of the discharge circuit, said portion being downstream from the discharge control valve and the discharge throttle valve, the bypass circuit including a stop valve having an opening/closing function, a flushing circuit for flushing the ram cylinder, the flushing circuit being established when the bypass circuit closes communication between the supply circuit of the hydraulic unit and the discharge circuit of the hydraulic unit and when the supply control valve and the discharge control valve are controlled to be in the open positions; and a driving circuit for driving the ram cylinder, the driving circuit being established when the bypass circuit establishes communication between the supply circuit of the hydraulic unit and the discharge circuit of the hydraulic unit.