NOVELTY - Residue after evaporation of injected sludge and digested sludge (TS), ignition loss (VS), residue after ignition (FS) are measured. Concentration (CN) of ammoniacal nitrogen (AN) is calculated from TS, VS and FS. CN of ammonium hydrogen carbonate is calculated from CN of AN. Organic substance ratio of digested and injected sludge are calculated and digestibility is computed.    USE - For evaluating anaerobic digestion.    ADVANTAGE - The method enables quick and accurate measurement of ammonium hydrogen density concentration, hence digestibility is measured accurately. The apparatus can be inexpensively maintained.    DETAILED DESCRIPTION - The method involves, process of measuring:    (a) residue after evaporation of injected sludge and digested sludge (TS);    (b) ignition loss (VS); and    (c) residue after ignition (FS).    Concentration of ammoniacal nitrogen (AN) in injected, digested sludge and, total inorganic carbon concentration are calculated from TS, VS and FS values. Concentration of ammoniacal hydrogen carbonate concentration is calculated from total inorganic carbon concentration and AN concentration. Reduced value is added with measured value of residue after evaporation of injected sludge. Ratio of organic substance in injected sludge is calculated from, ignition loss value of injected sludge, measured value of residue after ignition and, the value obtained by adding reduced and measured value of TS. The sludge is passed through second measurement unit and TS, VS, FS are measured. Ammonium hydrogen carbonate concentration is measured from ammoniacal nitrogen concentration of digested sludge and total inorganic carbon concentration. The reduced value is added with measured value of residue after evaporation of digested sludge. The organic substance ratio of digested sludge is calculated from, ignition loss of digested sludge, added value of reduced and measured value of residue after evaporation of digested sludge and, residue after evaporation. The digestibility is calculated from the measured value of organic substance ratio of injected and digested sludge.    An INDEPENDENT CLAIM is also included for digestibility measuring apparatus.