A control method of an internal-combustion engine having a turbo supercharger with a variable nozzle capable of adjusting a flow velocity of gas exhausted to a turbine impeller, the method executed by a control apparatus comprising the steps of: calculating a fuel flow rate based on an injection quantity of fuel in the internal-combustion engine and a number of revolutions of the internal-combustion engine; calculating an upstream side pressure on an upstream side of the turbine impeller; calculating an upstream side temperature on the upstream side of the turbine impeller; calculating a turbine flow rate of gas flowing toward the turbine impeller; calculating an upper limit threshold value of an opening degree level of the variable nozzle, wherein an opening degree of 100% is a completely closed state and of 0% is a completely open state of the variable nozzle, such that a turbine pressure ratio is equal to or less than a predetermined ratio based on the turbine flow rate and a pressure ratio turbine flow rate characteristic, wherein the turbine pressure ratio is a ratio of a pressure on the upstream side to the pressure on a downstream side of the turbine impeller, wherein the pressure ratio turbine flow rate characteristic is pre-stored information indicating a relationship between the turbine pressure ratio, the turbine flow rate, and the opening degree level of the variable nozzle; controlling an opening degree of the variable nozzle so as to be the opening degree equal to or less than the upper limit threshold value, wherein the control apparatus calculates the upstream side pressure based on an intake air flow rate received in the internal-combustion engine, the fuel flow rate, and an atmospheric pressure, wherein the control apparatus calculates the upstream side temperature based on the injection quantity, the intake air flow rate, and the fuel flow rate, and wherein the control apparatus calculates the turbine flow rate based on the fuel flow rate, the intake air flow rate, the upstream side pressure, and the upstream side temperature. calculating a fuel flow rate based on an injection quantity of fuel in the internal-combustion engine and a number of revolutions of the internal-combustion engine; calculating an upstream side pressure on an upstream side of the turbine impeller; calculating an upstream side temperature on the upstream side of the turbine impeller; calculating a turbine flow rate of gas flowing toward the turbine impeller; calculating an upper limit threshold value of an opening degree level of the variable nozzle, wherein an opening degree of 100% is a completely closed state and of 0% is a completely open state of the variable nozzle, such that a turbine pressure ratio is equal to or less than a predetermined ratio based on the turbine flow rate and a pressure ratio turbine flow rate characteristic, wherein the turbine pressure ratio is a ratio of a pressure on the upstream side to the pressure on a downstream side of the turbine impeller, wherein the pressure ratio turbine flow rate characteristic is pre-stored information indicating a relationship between the turbine pressure ratio, the turbine flow rate, and the opening degree level of the variable nozzle; controlling an opening degree of the variable nozzle so as to be the opening degree equal to or less than the upper limit threshold value, wherein the control apparatus calculates the upstream side pressure based on an intake air flow rate received in the internal-combustion engine, the fuel flow rate, and an atmospheric pressure, wherein the control apparatus calculates the upstream side temperature based on the injection quantity, the intake air flow rate, and the fuel flow rate, and wherein the control apparatus calculates the turbine flow rate based on the fuel flow rate, the intake air flow rate, the upstream side pressure, and the upstream side temperature.