A welding apparatus provided with lower electrode portions, upper electrode portions, and a cooling water circulation duct which circulates and supplies cooling water to welding electrode pressurizing cylinder guns on the upper electrode portions. A check valve is disposed on an upstream side to the cylinder guns in the cooling water circulation duct. Further, a booster is disposed on a downstream side to the cylinder guns in the cooling water circulation duct, and the cylinder guns are extended for welding by pressurizing the cooling water in the duct from the booster to the check valve through the cylinder guns.