PURPOSE: To obtain a galvanic batter type combustible gas sensor which can operate even in a combustible atmosphere containing no oxygen.

CONSTITUTION: This apparatus has an electrolyte 4, a positive pole 2 which is arranged in the electrolyte 4 to reduce oxygen dissolved in the electrolyte 4 electrochemically and has at least one catalyst selected from a group comprising a gold catalyst, a carbon catalyst and a platinum catalyst and a negative pole 3 which functions as gas pole and has a catalyst effective for electrochemical oxidizing reaction of a combustible gas. In the positive pole 2, a reducing reaction alone of the oxygen dissolved in the electrolyte occurs and in the negative pole 3, an oxidizing reaction alone of the combustible gas occurs thereby enabling sufficient operation even in a combustible atmosphere containing no oxygen.