The anaerobic decomposition of sludge wasinvestigated. With the decrease of ORP, pH increased and MLSS decreased. BOD value ofsupernatant was larger than COD value, butthe ratio of both values was not constant. Acetic and propionic acids were detected in alarge amount as volatile organic acids, and the ratio of volatile acids to BOD values was not constant. As N compounds, NH3-N wasmuch, but NO2 and NO3-N were containedalittle. N content in dry sludge was 6.46% and corresponded to 40% as protein content. As Pcompounds, orthophosphate from was much and organic form was a little. P content in dry sludge was 0.373%. As sulfides in sludge, comined form was much and free form was a little, while a largeamount of sulfate-S was contained in filtrate. For gas production by unacclimatized sludge, the large production periods were observed at four times. CH4 and N2 were much, while H2 and H2S were a little. If digested sludge was mixed, the gas production decreased andand only one production period was observed.