PURPOSE: To reduce the cost of a device and to improve reliability by using a built-in uniform-pressure mechanism consisting of a uniform-pressure path, a connecting hole, a gate valve, and a stop valve for performing zero-point check.

CONSTITUTION: A high-pressure side uniform-pressure path 31 where one terminal is opens at a high-pressure-side pressure introduction room 11 and a low- pressure side uniform-pressure path 32 which opens at a low-pressure side pressure introduction room 10 are provided. Also, the other terminal of the path 32 and that of the path 31 are connected and a gate valve 35 for opening and closing the connection hole 33 is provided. Further, a high-pressure side pressure- guiding pipe 36 with a high-pressure side stop valve 38 halfway and a low- pressure side pressure-guiding pipe 37 with a low-pressure side stop valve 39 are connected to introduction ports 5 and 4. Then, first the gate valve 35 is loosened to cause the path 31 and the path 32 to continue. Then, pressure applied to seal diagrams 12 and 13 is made uniform after closing both stop valves 39 and 38 and then zero-point check is made, thus reducing the cost of a device and at the same time improving the reliability in instrumentation by reducing the number of points where a measurement liquid leaks.