To provide a flow rate measurement system and a flow rate control method for optically measuring flow rate in a flow channel provided with a flow rate controller.SOLUTION: A flow rate measurement system 100 includes: a measurement cell 22 which is interposed on a flow channel at a downstream side of a flow rate controller 10 and in which gas flows; a light source 24a for generating light to be incident on the measurement cell; a photodetector 24b for detecting intensity of the light incident on the measurement cell from the light source and passing through the measurement cell; a downstream valve V2 provided downstream of the measurement cell; and a processing circuit 24d connected to the photodetector. The processing circuit calculates flow rate Q of the gas on the basis of change in the intensity I of the light passing through the measurement cell measured by the photodetector after closing the downstream valve V2 from a state in which the gas flows at the flow rate controlled by the flow rate controller 10.SELECTED DRAWING: Figure 1