The actual flow rate of gas to the solid-sample container or to the gas reservoir can be precisely obtained according to variances in the pressure of the gas reservoir without directly adjusting the flow rate precisely. The volume of gas not adsorbed in the sample container can be obtained from the pressure in the sample container. The adsorption can easily be determined from the difference between the flow rate of gas and the volume of gas not adsorbed in the sample container. Even if the flow rate fluctuates, the adsorption can precisely be obtained. The desorption can easily be obtained in the same way. Gas is exhausted from the gas reservoir in advance. When desorbed gas in the sample container is continuously exhausted to the gas reservoir, the pressure in the gas reservoir is measured. Since the adsorption and desorption are thus precisely obtained, the adsorption or desorption isotherm can be drawn by plotting the pressure at each point of time and the corresponding adsorption or desorption of the solid sample. Consequently, the surface area, pore diameter distribution, pore volume and the like of the solid sample can precisely be calculated.