PURPOSE: To obtain a highly reliable displacement detector which eliminates the need for close maintenance and is used suitably for nuclear reactor facilites by leading the motion of a relief valve system to a cylinder for pressure detection and installing a pressure reception part in piping laid out of severe environmental conditions.

CONSTITUTION: A displacement detection piston 40 coupled directly with a stem 30 at the upper driving part 3 of a main steam relief safety valve 13 and the pressure reception part applied with the pressure of reciprocation are connected together by detection piping 42 wherein sealing liquid 41 is enclosed through a sealing port 40. Further, the pressure reception part 43 and an iron core 37 are associated operatively and the displacement detector 35 converts a signal into an electric signal 48.