One-way valve that prevents backflow of hydraulic fluid in hydraulic systems. Used in hydraulic circuits of press machines, injection molding machines, and machine tools for maintaining cylinder holding pressure, preventing pump discharge side backflow, and maintaining accumulator pressure. Available in spring-type, pilot-type, and swing-type structures, selected based on cracking pressure, maximum flow rate, and connection size (G1/4, G1/2, G3/4, etc.). Unlike relief valves or counterbalance valves, this hydraulic component specializes in simple backflow prevention function.