PURPOSE: To make it possible to simply measure the surface area of a specimen, by such a simple mechanism that an adsorbing amount is calculated from the difference between values before and after the pressurization of inert gas calculated by a formula (pressure×volume/temp.) and the surface area of the specimen is calculated from said adsorbing amount.

CONSTITUTION: A specimen 4 to be measured is inserted in the specimen container 3 within a manifold 5 and subjected to degassing treatment under heating and, thereafter, the specimen container 3 is immersed in liquid nitrogen 9. Next, the specimen container 3 is evacuated and inert gas is introduced into said container 3 (to set pressure P1). A piston 2 is moved to reduce the pressure P1 to pressure P2 and the volumetric increase amount at this time is set to ΔV, the volume of the manifold 10 to Vd, temp. to T and the temp. of liquid nitrogen to TN2 to calculate a dead volume Vs from formula I. Subsequently, inert gas is introduced at relative pressure (x) of about 0.3 and the position of the piston 2 is adjusted so as to obtain 4W5 measured values in a relative pressure range of 0.05W0.3 and an adsorbing amount V is calculated from the difference between values before and after pressurization calculated by a formula (pressure×volume/temp.) and a monomolecular adsorbing amount is calculated by formula II and the surface area S of the specimen is calculated from formula III.