A control circuit (20) of a mass flow control device (1) retains the relation (Cd=f(ReTH)) between the Reynolds number and the actual discharge coefficient of a sonic nozzle (13) in the area where the Reynolds number is small. In such an area, the sonic nozzle can be used as a variable mass flow element for controlling mass flow rate. If a pressure Pu and a temperature Tu of an upstream fluid of the sonic nozzle 13 are detected, the theoretical mass flow rate QmTH and the theoretical Reynolds number ReTH can be calculated; since the actual discharge coefficient Cd can be calculated by the above relationship Cd=f(ReTH) based on the calculated Reynolds number, the actual mass flow rate Qm can be calculated based on the relation of Qm=Cd.multidot.QmTH. In the control circuit 20, in order to obtain the predetermined mass flow rate Qm, the upstream fluid pressure Pu and temperature Tu of the sonic nozzle are obtained based on the above relationship, and a variable valve (12) is driven so that the mass flow rate may have this value. Since the properties of the sonic nozzle can be made full use of, it is possible to control trace amounts of mass flow rate with extremely high accuracy.