PRESSURE RELEASING VALVE AND VISCOUS COUPLING

PROBLEM TO BE SOLVED: To provide a pressure releasing valve of which manufacturing can be carried out through an easy assembling work and provide a viscous coupling having the pressure releasing valve.SOLUTION: This pressure releasing valve 21 is constituted such that the same comprises a piston valve body 24 moved between a closed position P1 and an opened position P2, a plug housing 23 storing the piston valve body, an annular seal body 25 composed of elastic material, and an engaging part 26 formed at the piston valve body 24 and holding the piston valve body 24 at a closed position P1 while becoming an engaged state to be engaged with the seal body 25. When a prescribed pressure is applied to the piston valve body 24, the seal body 25 is elastically deformed with a pushing force generated between the engaging part 26 and the seal body 25 to release the engaged state, thereby the piston valve body 24 is moved to an opened position P2 to cause a flow passage 27 to become a released state and further when the seal body 25 is heated up to a prescribed temperature under a state in which the piston valve body 24 is kept at the closed position P1, the flow passage 27 becomes a released state through a clearance generated under thermal deformation of the seal body 25.