NOVELTY - A relationship between pressure of stored purified gas and injection rate of component-adjustment gas to be injected into the town-gas pipe (28) is previously calculated to ensure preset flow volume and preset minimum pressure of purified gas utilized by the installation. When supplying purified gas to the utilization installation, pressure of stored purified gas is measured. The injection rate of component-adjustment gas to the town-gas pipe is increased/decreased by the injection-rate adjustment unit from the relationship.    USE - Control of injection rate of component-adjustment gas in town-gas pipe in biogas utilization system.    ADVANTAGE - The method ensures specific flow volume and minimum pressure of purified gas utilized by the utilization installation in a biogas utilization system.    DETAILED DESCRIPTION - The injection rate of a component adjustment gas in town-gas pipe (28) in a biogas utilization system provided with a biogas purifier (2) which purifies high-concentration methane gas by separating sulfur-type impurity from biogas, a medium-pressure gas holder (3) which stores the purified gas, a utilization installation to which the stored purified gas is supplied, a component-adjustment unit which adjusts the stored purified gas to preset composition and concentration which town gas requires, a piping (29) for guiding component-adjustment gas to the preset composition and concentration to the town-gas pipe, an injection-rate adjustment unit, and a pressure gauge (4) for measuring the pressure of stored purified gas, in the midstream route of piping, is controlled as follows. A relationship between pressure of stored purified gas measured by the pressure gauge, and injection rate of component-adjustment gas to be injected into the town-gas pipe is previously calculated to ensure preset flow volume and preset minimum pressure of purified gas utilized by the installation. When supplying purified gas to the utilization installation, pressure of stored purified gas is measured. The injection rate of component-adjustment gas to the town-gas pipe is increased/decreased by the injection-rate adjustment unit from the relationship. An INDEPENDENT CLAIM is included for injection-rate control apparatus provided with a memory unit in which the calculated relationship is stored, and an injection-rate calculation unit for computing the injection rate of component-adjustment gas.    DESCRIPTION OF DRAWING(S) - The drawing shows an explanatory view of the biogas utilization system. (Drawing includes non-English language text)    Biogas purifier (2)    Medium-pressure gas holder (3)    Pressure gauge (4)    Town-gas pipe (28)    Piping (29)