PURPOSE: To utilize efficiently the digester gas produced in a sewage treatment or the like and to improve the operating time for a power generation installation with digester gas by controlling the flow rate of the digester gas to be supplied by taking the quantity of heat required for heating a digester into consideration.

CONSTITUTION: The temp. tI and flow rate Q of the sludge to be charged into a digester 1 and atmospheric temp. tA are measured respectively with detectors 12, 13, 14, and are inputted to an arithmetic device 18, which calculates the quantity of heat H required for heating the digester in accordance with the equation I . The quantity of heat H and the amt. of the digester gas to be produced measured with a detector 15 are inputted to an arithmetic device 19, and the amt. FE of the digester gas that can be supplied to a power generation installation 5 is calculated by the equation II (ηB: the efficiency for heating the digester, ηG: the heat recovery rate of the power generation plant, D: the calorific value of the digester gas). The flow rate of the digester gas to be supplied to the installation 5 is controlled in accordance with the amt. FE and the pressure P of a gas tank 4 in which the digester gas is stored, whereby the number of the gas engines to be operated is determined.