Some of the experimental conditions of JIS A 1905 "Performance test of adsorptive building materials of reducing indoor air pollution with small chamber" are different from the actual building environment. The agreement between a decreasing effect resulted from JIS test method and that under residential indoor environment is suspected. The optimum experimental conditions resulting true effect were investigated with an experimental design. Within the scope of the performance test with 0.15m3 volume of a SUS small test chamber and a specimen of adsorptive building material in this study, appropriate experimental conditions are verified, namely optimum loading factor is 0.5m2/m3 and optimum supply HCHO concentration is 100μg/m3. The decreasing effect of a specimen resulted from a test under this condition is suited to the effect under actual condition.