PURPOSE: To reduce the errors in heat condition, by making the temperatures in the vicinity of a temperature contact point uniform as much as possible.

CONSTITUTION: The thermocouple 2 comprising two strand wires (a) and (b) is inserted in the protecting pipe 1, and the contact point of the strand wires (a) and (b) is welded to a point P at the one end of or in its vicinity of the protecting pipe 1. The strand wire (a) is welded to a point Q which is slightly separated from the end point P of the protecting pipe 1 (e.g. separated by one cm). A heating wire 3 for heating the part of the point Q of the protecting pipe is wound around the opposite side of the P with respect to the Q. The temperature difference between the two points P and Q can be detected by the thermocouple constituted by the strand wires (a) in the inside and at the outside of the protecting pipe and the part PQ of the protecting pipe 1. The current through the heating wire 3 is controlled accordingly so that the temperature difference becomes zero. Thus the errors in the heat conduction can be reduced to the large extent. The temperature is measured by the thermocouple in the protecting pipe.