PURPOSE: To improve measuring accuracy of powdered body having ≥500m2/g specific surface area, by constuting a sample tube with anhydrous quartz containing ≤1ppm volatile components in a measuring apparatus of the specific surface area performing by an adsorption method.

CONSTITUTION: A gaseous N2 11 adsorbed to a sample and a gaseous He 12 not adsorbed to the sample are mixed in a proportional mixing tank 13 and gaseous mixture is flowed at a constant flowing speed and is cooled in a cold trap. Then, said gas is passed through a TC detector 15 and the gaseous N2 11 is adsorbed to the sample in a sample tube 16. The tube 16 is made of anhydrous quartz containing ≤1ppm volatile components. After reached adsorption equilibrium, the gaseous N2 is released because of heating the tube 16 at a normal temperature again and a peak of released gas is recorded on a recorder 18 after passing through the detector 15. A known volume N2 calibration gas 19 is introduced into the tube 16 for carrying out calibration and the peak is recorded and then, the quantity of the gaseous N2 adsorbed to the sample is found by comparing each peak. A specific surface area is measured by changing mixing composition of gaseous N2 and He and repeating the same process.