This invention provides a diagnostic mechanism of a differential pressure type mass flow controller comprising a diagnostic parameter calculating section that obtains a mass flow rate integrated value by means of an integrating calculation from the lowering pressure value of an inlet side sensor among the inlet side sensor and an outlet side sensor arranged in communication respectively at the inlet side and the outlet side of a differential pressure generating resistive element that generates a differential pressure between the inlet and the outlet by changing a flow rate control valve arranged on the channel where a fluid flows from a flow rate control state to a closed state, and further obtains a diagnostic volume value from the obtained mass flow rate integrated value, and a comparing section that compares the diagnostic volume value obtained at the diagnostic parameter calculating section with a specified volume value.