PURPOSE: To prevent the injection of a pressure medium when a pressure detection part is destructed, by moving a valve body when the difference between the pressure of a pressure introducing side and that of a pressure detection side reached specific pressure difference to seal a pressure detection path by said pressure difference.

CONSTITUTION: If it is assumed that diaphragm 23 was destructed by the abnormal rising in pressure to be detected in a use state, a pressure medium is injected from the destructed diaphragm 23 and the pressure in the side of a pressure detection path 16 is abruptly reduced. With the reduction in pressure, pressure to be detected allows a valve body 13 to contact with an elastomer 17. The longitudinal groove 15 of the valve body 13 is provided only to the periphery of the valve body 13 by engraving and the pressure detection path 16 is sealed by the elastomer 17 and the valve body 13 because the valve body 13 is contacted with the elastomer 17 and pressed by pressure to be detected and the pressure medium can be prevented from flowing to the side of the pressure detection path 16 from the communication hole 18 of the elastomer 17.