an intake system control device includes: an acquisition section that acquires an operation condition of an engine; a steady-state value determination section that determines input and output steady-state values for an intake system of the engine based on the acquired operation condition; a gain determination section that determines a gain for a 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 an intake system output by using the state space model having the determined gain.