To provide a relief valve improved in durability regardless of its low cost and simple structure.

A reciprocation trajectory of a valve body 10 is made constant, and a passage for oil is secured, by forming a guide piece 16 which slides on an inner wall surface 21a of an oil passage 27 during a reciprocating action in opening and closing, and has a slit 18 communicating a valve hole 22 with an oil discharge opening 25, on the valve body 10 energized by a coil spring 40 and opening and closing the valve hole 22 of a seat part 23 in response to an oil pressure of an oil inlet 24.