For the stabilization and reduction of the excess activated sludge produced in a small-scale wastewater treatment plant, an aerobic digestion process is considered to be more suitable than an anaerobic one. In this paper, batch aerobic digestion experiments with and without pH adjustment to 7 were investigated using acclimated sludge, which was acclimated to synthetic wastewater (substrates: glucose and peptone), and excess activated sludge, which was taken from a municipal wastewater treatment plant. The pH adjustment did not greatly affect the final ratio and the rate of decomposition of the sludge solids. The final decomposition ratio of the excess activated sludge was smaller than that of the acclimated sludge, but the decomposition rate of the excess activated sludge was larger than that of the acclimated sludge. The accumulated amount [mol] of NaOH applied in order to adjust pH to 7 was almost equal to the amount of sludge decomposition [mol-C5H7NO2], and it was confirmed that the aerobic digestion occurred according to the predicted reaction. Therefore, the degree of sludge biomass decomposition can be estimated from the amount of applied alkali chemicals. Since the pH adjustment to 7 promotes the nitrifying reaction, NH4-N is rapidly oxidized to NO3-N, and very little NH4-N remaines in the liquid phase. The phosphorus concentration of the liquid also decreased, because the orthophosphate dissolved into the liquid phase from biomass is precipitated as an apatite by the pH adjustment to 7. The production of hydragen sulfide gas in the decomposition of the sludge, was. determined by the under of days before the darkening of lead acetate paper. It was faund that the production of the gas was delayed when pH was adjusted to 7, and that the adjustment repudly stabiliged the digested sludge.