To provide mass flow rate controller which enables high-accuracy assay by reflecting both pressure and mass flow and also allowing for variation in tank volume.

This is the method for assay of the mass flow rate controller which comprises a valve for assay 42 installed in utmost upstream of channel 6, mass flow rate control valve mechanism 10; tank 44 mounted on the channel 6 of upstream from the mass flow rate control valve mechanism 10; mass flow detecting means 8, pressure detecting means 46, means 18 for controlling the mass flow rate control valve mechanism 10, and mass flow rate assay control means 48. The method includes (1) streaming the fluid of a defined mass flow rate to the channel 6, (2) for maintain opening of the mass flow rate control valve mechanism 10 so as to match the mass flow of the fluid with the defined mass flow; (3) closing the valve for assay 42, (4) for measuring pressure and mass flow rate of the fluid after flow of the fluid running out from the tank 44 is stabilized; (5) calculating the rate of change in pressure and mass flow rate with respect to the reference pressure and mass flow measured by the same procedure in the initial state, and then (6) conduct assay, depending on the rate of change.