To stably supply a gaseous fuel by setting a calorific value as a management value even if a composition of the fuel varies. A composition-independent flow meter 4 is disposed in series with a thermal flow meter 3 in a supply pathway of the fuel. When the measured values of the both flow meters 3,4 differ by a certain degree, it is determined that there is an abnormal state, and the conversion factor with respect to the thermal flow meter 3 is set. The composition of the fuel is estimated based on the conversion factor, and the calorific value of the fuel is estimated based on the composition. The supply amount of the fuel by a supply device 2 is adjusted based on the calorific value of the fuel so that the target supply calorific value per unit time is obtained.
