To meet the increased demand of porosity evaluation of low-k materials, the ultra-high-sensitive Kr gas adsorption technique was developed. A homemade differential-type adsorption apparatus enables us to detect Kr adsorption as small as 2 mug. This corresponds to the sensitivity improvement by four orders of magnitude compared to the conventional nitrogen adsorption method. Established analytical procedures for the pore-size distribution, Brunauer-Emmett-Teller. theory, Kelvin equation, and Dollimore-Heal method, are applied to Kr adsorption. The results were calibrated with one obtained from nitrogen adsorption using standard samples. Applicability of the Kr adsorption method to a low-k thin-film sample was demonstrated.