To provide a galvanic-cell-type gas sensor, capable of being formed thin by forming an anode of a size corresponding to a sensor life.

A cathode 2 arranged in the back surface of a diaphragm 8 with water repellency which allows the inflow of a gas to be detected, the anode 6 constituted by forming a lead layer by electrolytic plating, in such a way that a part of a conductive material with corrosion resistance to an electrolytic solution remains as a lead 7, and a sheet 5 inserted between the cathode 2 and the anode 6 and impregnated with an electrolytic solution are housed in conductive cases 13 and 18, in such a way as to cause the diaphragm 8 to be exposed.