PURPOSE: To improve reliability/safety by stably, speedily and safely controlling the quantity of an additional component such as smelled liquid to be additionally injected by controlling the rotating number of revolution of a pump or the aperture of a valve so that the deviation of a flow rate can be zero.

CONSTITUTION: A relation G between a 'pump output or valve aperture' F and the flow rate of fluid C is found in advance (1), the flow rate of fluid A is measured (2) and a flow rate D of fluid C required for turning the ratio of fluid C to be injected into the flow rate of fluid A to B is calculated (3). Based on this flow rate D, an output signal Z from the relation G to the rotation of the pump or the valve aperture is instantaneously outputted (4), a current flow rate E of the fluid C is measured (5), and the number of revolution of the pump or the valve aperture is controlled so that the deviation between the flow rates D and E can be zero (6). Therefore, since feedback control with a controllable range limit is utilized for the output signal value Z, the quantity of the additional component such as smelled liquid to be additionally injected to the gas flow of town gas can be stably, speedily and safely controlled.