A recording means records a value of pressure of fluid on a downstream side of a differential pressure generation means during a time period after a flow control valve open and control of a flow rate is started or after the flow control valve is closed and the control of a flow rate is stopped until a measured flow rate becomes stable, and a diagnostic means contrasts a controlled value acquired based in a value of the pressure of the fluid on the downstream side recorded by the recording means, with a controlled value when abnormality has not occurred, and diagnoses that abnormality has occurred in the flow meter when a difference between the both controlled values exceeds a threshold. Thereby, a mass flow controller and a diagnostic method which can simply diagnose generation of abnormality in a differential pressure type flow meter without requiring time only for performing the diagnosis.