A sensor for detecting a carbonic acid gas dissolved in body fluids has a laminated structure including an outer plate having a carbonic acid gas permeable window, a plate for a pH-electrode having a pH sensitive membrane, an intermediate plate having a cavity for accommodating an electrolyte, and a plate for a reference electrode. The plate for the pH-electrode is constituted in a manner that a pH sensitive membrane constituted by a membrane of an oxide of a platinum group metal is formed on an insulating substrate and the electrolyte contacts with the pH sensitive membrane in a groove. The pH sensitive membrane is disposed so as to oppose to the carbonic acid gas permeable window.