Instead of a single-element sheathed thermocouple, a sheathed thermocouple is used in which: two pairs of thermocouple wires are enclosed in a metal sheath while being embedded in an inorganic insulating material powder, the thermocouple wires being made of the same materials as and having a diameter that is 90% or greater of or substantially equal to the diameter of thermocouple wires of the single-element sheathed thermocouple, and the metal sheath being made of the same material and having the same outer diameter and wall thickness as a metal sheath of the single-element sheathed thermocouple; only one pair of the two pairs of thermocouple wires are joined to each other at leading ends to form a measuring junction; and a base end of the metal sheath is sealed with a seal with the pair of thermocouple wires joined to each other at the leading ends passing therethrough.