A temperature varying low-temperature comparative calibration including a vacuum jacket, a comparative calibration block in the vacuum jacket which is formed of a material of a high heat conductivity and which is maintained at a fixed temperature by a refrigerator or a heater, a reference thermometer housing portion in the comparative calibration block in which a reference thermometer is mounted, and an insertion hole in the comparative calibration block through which a calibration target thermometer is inserted from outside the vacuum jacket via an introduction pipe. The introduction pipe is filled with heat exchange gas.