To provide a diagnostic method for a gas flow controller which can determine the presence/absence of offset of a flowmeter or deviation of opening of a control valve even while gas is being supplied.

When gas is supplied from a gas generation source through a flow controller to facilities disposed on the downstream side of the flow controller by a necessary amount of gas, gas pressure (P0) on the upstream side of the flow controller and gas pressure (P1) on the downstream side and an indicated value (f) of the flowmeter are shown, the indicated value (Mv) of the valve opening of the flow control valve whose valve body rated value is Cv is measured at two time periods when the flow rates are different, and they are substituted for equations: F= Cv×(S/100)2×√(P0-P1) (1), F=f-δf (2), and S=Mv-δm (3). After thus setting up six simultaneous equations in total, a deviation (δf) between the indicated value of the flowmeter and the true value (F) of the flow rate, and a deviation (δm) between the indicated value of the valve opening and the valve opening, are computed, and the deviations (δf) and (δm) obtained by the computation and their respective allowances are compared.