PURPOSE: To obtain arbitrary ref. temp. with high accuracy and achieve microminiaturization by setting the cooling surface of an electronic cooling element at prescribed temp.

CONSTITUTION: This is provided with a vacuum vessel 10 which is tightly closed, an electron cooling element 13 of which the heating surface is closely fitted and fixed to the base 12 of the vessel 10, a temp. control element 14 which is closely fitted and fixed to the cooling surface of said element 13, and outputs the signal corresponding to the deviation from preset temp. by detecting the temp. of the cooling surface, a temp. calibrating thermocouple 20 of which the mutual connection points 20B, 20C of thermocouple element 21, 22 and output wires 23, 24 are abutted and fixed on and to the cooling surface 13b, and an electric power source circuit 25 which controls the electric curent to be supplied to the cooling element 13 in accordance with the deviation signal. The cooling surface 13b of the element 13 is set at prescribed temp.