PURPOSE: To provide such a fine periodic variation in temperature during cooling operation as one which can be ignored in fact by a method wherein a second gas injection part for leaking cooling gas from a fine hole at the extremity end of a fine tube for guiding the cooling gas under a closed condition of a first gas injection part while the fine hole is placed near and opposite to the second gas injection part.

CONSTITUTION: A gas injection port 4a and a needle 10 are provided as a first injection part 4, and there are provided second injection parts 111 and 112 for directing liquified gas 15 toward a sensor element 14 and injecting it through a wall of an inner tube 11 even if the first injection part 4 is closed. That is, a guiding tube 100 for transporting liquified gas to a place near the sensor element, fine gas injection ports 101a and 101b at the extremity end of the guiding tube are placed near and opposite to the sensor element through the wall of the inner tube 11 and further placed so as to cause the liquified gas to perform a direct striking against the bottom part of the sensor element, resulting in composing the second gas injection parts 111 and 112. With such an arrangement as above, it is possible to make such a fine variation of cooling temperature as one in which it can be ignored.