PURPOSE: To provide the liquid addition rate control system which conforms with variation in performance characteristics of a pump and makes disturbance hard to affect a flow rate.

CONSTITUTION: An odorization controller 7 outputs stroke signals to diaphragm pumps 3a-3c. Consequently, the odorizing agent stored in a tank 2 is injected into the gas in gas pipes 1a-1c through pipes 8, 8a-8c, and 4a-4c. The flow rates of the odorizing agents sent from the diaphragm pumps 3a-3c are measured by Coriolis force flowmeters 6a-6c. The odorization controller 7 calculates, for example, the tertiary moving means of momentary flow rate signals outputted form the Coriolis force flowmeters 6a-6c at intervals of sampling time to find the mean momentary flow rate. The odorization controller 7 feeds this mean momentary flow rate back, and operates the stroke signals outputted to the diaphragm pumps 3a-3c according to an addition rate set value, the flow rate of the gas, and the mean momentary flow rate to control the flow rates of the odorizing agents sent from the diaphragm pumps 3a-3c to the addition rate set value.