PURPOSE: To make it possible to obtain a good hydrogen sensor element, by stable baking using a refractory substance based on oxide of an element belonging to the Group IIa of the Periodic Table as a baking jig.

CONSTITUTION: In forming a high temp. hydrogen sensor element using a proton conductive solid oxide sintered body based on strontium and cerium, a refractory substance based on oxide of an element belonging to the Group II a of the Periodic Table is used as a baking jig. When the baking jig comprising electrolytic zirconia, electrolytic alumina, electrolytic mullite, zircon, chromium or silicon is used, the reaction with strontium of the sensor element sintered body is generated to form a compound and the separation of ceo is measured by a powder X-ray method. As the oxide of the element belonging to the Group IIa, for example, magnesia, calcia or baria are represented and, when a refractory substance, which is based thereon and contains 3% or less inevitable impurities, is baked, reactivity is not observed at all and a sintered body is obtained.