PURPOSE: To reduce the cost of an element by bringing the outside electrode layer of the element into direct press contact with a plug body to maintain an electrical contact therewith and forming the inside electrode layer thereof into a shape having no conductivity with the plug body.

CONSTITUTION: The plug body 4 is screwed to the gas chamber wall 10 of a measuring gas chamber 2 by a screw part. The element 3 is caulked to the part of the body 4 projecting into the chamber 2, by which the standard gas and measuring gas of a detector 1 are hermetically shut off. The outside electrode layer 8 made by calcining platinum powder, etc. is formed on the outside surface of the element 3. On the other hand, the inside electrode layer 9 is adhered to the inside surface of the element 3 in the same manner as for the layer 8. The element 3 has no electrode layer on the outside peripheral part and is made into the construction in which the layer 9 and the body 4 are insulated by the element 3. A terminal 5 is fixed to an insulator 11 for the terminal and the electromotive force generated in the layer 9 is conducted through a lead 14 by an adhering part 13 to the terminal 5. The electromotive force is thus generated between the layers 8 and 9 when the concn. of the hydrogen or steam in the gas in contact with both surfaces of the element 3 is different. The generated electromotive force is conducted to the body 4 and the terminal 5, by which the concn. of the hydrogen or steam is detected.