NOVELTY - A calculator (11) calculates the chemical oxygen demand volume (COD) load, by dividing the product of substrate input supplied to a methane fermentation tank and a substrate COD concentration with the volume of the tank. The ratio of methane conversion is calculated, by dividing the methanation ratio to COD volume load, which is compared with a standard value. DETAILED DESCRIPTION - Organic substrate is supplied to the methane fermentation tank (1) through the substrate opening (2). The methanation velocity is measured, by dividing the product of measured flow rate and methane concentration with the effective volume of the tank, which are measured by a gas flow meter (4) and a methane densitometer (5) provided between the tank and a gas holder. The volatile suspended solid concentration (VSS) of the sludge in the tank is measured by a sludge densitometer (6) and VSS concentration measuring device (7). The methanation ratio is calculated by a calculator (8). The rate of substrate inflow, chemical oxygen demand (COD) are measured by a flow meter (9) and COD measuring device (10). The COD volume load is calculated by a load calculator (11) and the ratio of methane conversion is calculated, which is compared with a standard value, by a comparator to judge the quality of methanation. An INDEPENDENT CLAIM is also included for methane conversion method.    USE -   For control of methanation in fermentation installation.    ADVANTAGE -   The ratio of methane conversion activity is measured, by the control devices during the fermentation, so as to get a favorable fermentation of the sludge. DESCRIPTION OF DRAWING(S) - The figure shows methane conversion control system of fermentation installation. (1) Fermentation tank; (2) Substrate inflow opening; (3) Gas holder; (4) Gas meter; (5) Methane densitometer; (7) VSS concentration measuring device; (8) Methanation ratio calculator; (9) Substrate flowmeter(10) Substrate COD measuring device; (11) COD volume load calculator; (12) Methane conversion ratio calculator.