Experimental study was carried out to clarify the effects of adsorbed substances on bioactivity of attached bacteria on GAC using ammonia oxidizing bacteria and m-aminobenzoic acid as an inhibitory adsorbed substance against ammonia oxidizing bacteria. Effects of adsorbed substances on GAC was determined by the activity of attached ammonia oxidizing bacteria in comparison with boiling stone with no capacity of adsorption. 50 % inhibition of ammonia oxidation were noted in m-aminobenzoic acid concentration of 60 mg.l−1 and 330 mg.l−1 on the GAC and the boiling stone columns, respectively. Inhibition of ammonia oxidation on GAC was clearly stronger than that on boiling stone, in spite of the same m-aminobenzoic acid concentrations in bulk solution between the GAC and the boiling stone columns. Bioactivity of attached bacteria was affected by adsorbed substances on GAC, when GAC was kept in equilibrium condition.