The cast iron coupler combined with two anchor bars is a joint device that can connect with two precast reinforced concrete elements easily and mechanically. In order to examine watertightness behavior of the joint with which two reinforced concrete beams were joined, tests for watertightness were carried out by a water-pressuring method. It was shown that watertightness of the joint was satisfacticable at service loading levels. And, the cast iron couplers can be used for precast reinforced concrete members even under the ground level.