PURPOSE: To avoid leaking of a material to be measured even under high pressure and to prevent the deformation of a cylinder by sealing the cylinder and pistons in a pressure container together with a compressing medium, and providing a compressing means for applying the pressure on the compressing medium.

CONSTITUTION: When a compressing medium 21 is compressed with a compressor 24 and a piston 23, the pressure is applied on a material to be measured 11 in a cylinder 12 through pistons 13a and 13b. The pressure on the material to be measured 11 is adjusted with the compressor 24 and measured with a pressure gage 15. The temperature of the material to be measured 11 is adjusted with a heater 14 and measured with a thermometer 16. The specific volume of the material to be measured is obtained by measuring the displacements of the pistons 13a and 13b with displacement gages 17a and 17b. In measurement, the pressures of the compressing medium and the material to be measured 11 are balanced through the pistons 13a and 13b. Therefore, the pressure difference between the inside and the outside of the cylinder 12 is substantially eliminated. Thus, the leakage of the material to be measured is avoided, and the cylinder 12 is not deformed.