In order to understand the characteristics of anaerobic digestion of the sewage sludge coagulated with inorganic coagulant before biological treatment, the batch type of anaerobic digestion experiments for three kinds of sewage sludge were performed. When the sewage sludge was coagulated by ferric chloride of 22mgFe/L, inhibition to anaerobic digestion was not observed, and reduction ratio of suspended solids and the amount of volatile fatty acids and methane gas increased compared with those of the sewage sludge without coagulant. Whereas, inhibition to anaerobic digestion was observed in sewage sludge coagulated by aluminum coagulants of 7.5mgAl/L.