To provide a liquid flow rate control method and its device which attains precise control of a flow rate concerning a liquid obtained by mixing not less than two liquid while properly adjusting concn., etc., unnecessitates complicated adjustment and does not reduce the purity of the liquid.

In this flow rate control method, ultrasonic sensors 5 and 5' are provided in flow passages 23 and 23' supplying liquid 2 and 2' into a processing tank 1 to detect a supplied liquid quantity by the ultrasonic sensors to compare the result between this detecting result and a previously stored supply quantity to control the quantity of a liquid to supply into the processing tank. In addition, the flow rate controller detecting and controlling the quantity of the liquid to supply into the processing tank is provided with a non-contact ultrasonic sensor with the liquid to monitor the liquid supply quantity into the processing tank by the sensor and provided with a controller 6 controlling the supplying quantity of the liquid within processing tank based on the data output of the sensor.