A hydrogen gas sensor is configured such that an O-ring 21 is disposed within a measurement chamber 11a in order to seal against a first support element 9a and against a proton conduction layer 1. This O-ring 21 assumes the form of a rectangular frame that surrounds a first electrode 3, and a substantially circular cross section, and is elastic, heat resistant, and durable. A first metal plate 23a and a second metal plate 23b are disposed from the outside on opposite support elements 9a and 9b, respectively. The hydrogen gas sensor of the present embodiment has a pair of right-hand and left-hand through-holes 29 formed therein so as to extend along the thickness direction thereof. Bolts 31 are inserted into the corresponding through-holes 29. By fastening the metal plates 23a and 23b from the outside by means of the bolts 31 and nuts 33, the opposite support elements 9a and 9b are fastened, thereby clamping the entire hydrogen sensor in a unitary condition.