A method of controlling a gas furnace comprising a gas valve for supplying a fuel gas to a manifold; a burner through which the fuel gas discharged from the manifold passes; an igniter for igniting a mixture of fuel gas passed through the burner and air; and an inducer for generating a flow in which a combustion gas generated by the burning of the mixture is discharged to an exhaust pipe via a heat exchanger, wherein the gas furnace performs a heating operation according to a heating signal or a heating stop according to a stop signal, includes the steps of: (a) receiving any one of the heating signal or the stop signal; (b) transmitting a signal to operate the inducer when the heating signal is received; (c) operating the igniter; (d) transmitting a signal to open the gas valve; (e) detecting whether the gas valve is opened or closed; (f) detecting a flow rate of the fuel gas in the manifold; and (g) displaying a normal operation of the heating operation, based on information detected in the steps (e) and (f).