PURPOSE: To measure directly hydrogen ion concn. under high temp. and pressure with a simple structure by providing a specified insulating body for sealing having a signal fetching terminal in the neighborhood of the opened end of a specified cylindrical body which is opened at one end and closed at the other end.

CONSTITUTION: A cylindrical body 1 is formed of an oxygen ion-conductive solid electrolyte consisting of stabilized zirconium which is doped with 8.5mol yttria, and one end is closed and the other end opened. An internal electrode 3 and an internal electrode 2 are housed in the cylindrical body 1. A metallic electrode 4 is buried in the cylindrical body 1, and the one end is exposed to the outside. A platinum wire 8 connected with the electrode 4 is buried in the cylindrical body 1. An insulating body 5 consisting of ceramic alumina is adhered to an adhering part 11 to seal the cylindrical body 1. The insulating body 5 is provided with terminals 6 and 7 which are connected respectively with the internal electrode 3 and the metallic electrode 4. Accordingly, the direct measurement of hydrogen ion concn. under high temp. and pressure can be made possible with the simple structure.