PURPOSE: To provide the function of a highly sensitive thermometer by a simple constitution, wherein a first sample chamber, in which liquified gas is sealed, and a second sample chamber, which is separated from the first sample chamber by differential pressure element and in which an absorbent is sealed together with the liquified gas, are provided.

CONSTITUTION: A sample chamber 2 is provided around a sample chamber 1. An absorbent 3 such as active carbon is sealed and a diaphragm type differential pressure element 4 is used. The diaphragm is deformed by the differential pressure. The deformation can be found highly accurately when the change in electric capacity between a fixed electrode and a moving electrode, which is attached to the diaphragm, is measured. A high pressure gas (e.g., 80 atom for helium) is added in the sample chamber 1 and the sample chamber 2 at a normal temperature, and the chambers are sealed and then cooled. The pressures in both sample chambers are shown by a solid line 5 and a dotted line 6. The output of a differential gage is about 0.8, which is maximum value, in the temperature range 7 (in the case of helium, 10K or more). The vapor pressure is outputted at 9 (in the case of helium, 5K or less).