An auxiliary feedwater system for maintaining a water level when the water level of secondary cooling water in a steam generator is not maintained in a main feedwater system, comprising: an auxiliary feedwater pipe connected to a subsequent stage of a main feed check valve in a secondary cooling water pipe; an auxiliary feedwater pump configured to feed auxiliary cooling water stored in an auxiliary feedwater pit to the auxiliary feedwater pipe; an auxiliary feedwater valve configured to adjust a flow rate of the auxiliary cooling water passing through the auxiliary feed water pipe; a water-level detection unit that detects a water level of the secondary cooling water in the steam generator; and a control unit configured to control the auxiliary feedwater valve based on the water level detection unit, including: a water-level-deviation calculation unit that calculates a deviation between a preset target water level and a water level detected by the water-level detection unit; a valve-operation setting unit that sets an aperture of the auxiliary feedwater valve corresponding to a deviation of the water level; and a valve drive unit that outputs a signal for driving the auxiliary feedwater valve corresponding to setting by the valve-operation setting unit, characterized in that the valve-operation setting unit included in the control unit is configured to: acquire water level time response information corresponding to an aperture of the auxiliary feedwater valve in advance, determine a parameter in a non-minimum phase transfer function, which approximates a response to the water level by an inverse response from a set aperture of the auxiliary feedwater valve, based on the water-level time-response information, and set the aperture of the auxiliary feedwater valve, taking into consideration a gain margin and a phase margin of frequency characteristics of the non-minimum phase transfer function in which the parameter has been determined. an auxiliary feedwater pipe connected to a subsequent stage of a main feed check valve in a secondary cooling water pipe; an auxiliary feedwater pump configured to feed auxiliary cooling water stored in an auxiliary feedwater pit to the auxiliary feedwater pipe; an auxiliary feedwater valve configured to adjust a flow rate of the auxiliary cooling water passing through the auxiliary feed water pipe; a water-level detection unit that detects a water level of the secondary cooling water in the steam generator; and a control unit configured to control the auxiliary feedwater valve based on the water level detection unit, including: a water-level-deviation calculation unit that calculates a deviation between a preset target water level and a water level detected by the water-level detection unit; a valve-operation setting unit that sets an aperture of the auxiliary feedwater valve corresponding to a deviation of the water level; and a valve drive unit that outputs a signal for driving the auxiliary feedwater valve corresponding to setting by the valve-operation setting unit, characterized in that the valve-operation setting unit included in the control unit is configured to: acquire water level time response information corresponding to an aperture of the auxiliary feedwater valve in advance, determine a parameter in a non-minimum phase transfer function, which approximates a response to the water level by an inverse response from a set aperture of the auxiliary feedwater valve, based on the water-level time-response information, and set the aperture of the auxiliary feedwater valve, taking into consideration a gain margin and a phase margin of frequency characteristics of the non-minimum phase transfer function in which the parameter has been determined. a water-level-deviation calculation unit that calculates a deviation between a preset target water level and a water level detected by the water-level detection unit; a valve-operation setting unit that sets an aperture of the auxiliary feedwater valve corresponding to a deviation of the water level; and a valve drive unit that outputs a signal for driving the auxiliary feedwater valve corresponding to setting by the valve-operation setting unit, characterized in that the valve-operation setting unit included in the control unit is configured to: acquire water level time response information corresponding to an aperture of the auxiliary feedwater valve in advance, determine a parameter in a non-minimum phase transfer function, which approximates a response to the water level by an inverse response from a set aperture of the auxiliary feedwater valve, based on the water-level time-response information, and set the aperture of the auxiliary feedwater valve, taking into consideration a gain margin and a phase margin of frequency characteristics of the non-minimum phase transfer function in which the parameter has been determined. acquire water level time response information corresponding to an aperture of the auxiliary feedwater valve in advance, determine a parameter in a non-minimum phase transfer function, which approximates a response to the water level by an inverse response from a set aperture of the auxiliary feedwater valve, based on the water-level time-response information, and set the aperture of the auxiliary feedwater valve, taking into consideration a gain margin and a phase margin of frequency characteristics of the non-minimum phase transfer function in which the parameter has been determined.