1. An intake system control device comprising: an acquisition section that acquires an operation condition of an engine; a steady-state value determination section that determines an input and output steady-state values for an intake system of the engine based on the acquired operation condition, the intake system being a system that includes an exhaust gas recirculation section, a variable nozzle turbocharger, a fresh air flow sensor, and an intake pressure sensor, receives as an input an intake system input including a valve opening degree in the exhaust gas recirculation section and a variable vane opening degree in the variable nozzle turbocharger, and outputs an intake system output including a fresh air flow rate measured by the fresh air flow sensor and an intake pressure measured by the intake pressure sensor, the input steady-state value being the intake system input obtained under the acquired operation condition and in the steady state, the output steady-state value being the intake system output obtained under the acquired operation condition and in the steady state; a gain determination section that determines a gain for a state space model, the state space model being a model that is used for control of the intake system, receives as a model input a variation of the valve opening degree and a variation of the variable vane opening degree, and outputs as a model output a variation of the fresh air flow rate and a variation of the intake pressure, the gain determination section determining the determined gain from the acquired operation condition and the valve opening degree based on a first relationship among the operation condition, valve opening degree, and a gain in the state space model; and a first input calculation section that calculates the intake system input from the acquired operation condition, the input steady-state value, the output steady-state value, and the intake system output by using the state space model having the determined gain.