A check valve has a valve body, a piston and a spring. The valve body has a tube, a nozzle and a valve seal. The tube has a sealing protrusion and an inner surface. The sealing protrusion is formed around the inner surface of the tube. The nozzle is combined with the tube and has an insert hole and multiple outlets. The valve seal abuts a shoulder of the sealing protrusion. The piston is mounted in the valve body and has a shaft, a sealing ring and a plug. The shaft is mounted through the insert hole. The sealing ring is mounted in the sealing groove. The plug is a conic frustum and is mounted on a plug end of the shaft. The spring is mounted around the shaft. In operation, the check valve regulates outflow volume under varying pressure to prevent a waste of water.