One-way valve that prevents backflow of hydraulic fluid in hydraulic systems. Used in hydraulic circuits of press machines, injection molding machines, machine tools, and construction machinery to ensure system safety and efficient operation as a critical component. Classified by structure into spring-type, pilot-type, and swing-type, with connection methods including threaded, flanged, and cartridge types. Unlike relief valves and directional control valves, it has no pressure control or flow control functions and performs purely backflow prevention. Essential for maintaining back pressure on the discharge side of hydraulic pumps, accumulator circuits, and cylinder circuits.