A second core calculates an intervening variable that is defined by an inner function, when a time derivative of a state variable is expressed by a function having the state variable, the inner function, and an observable input variable as independent variables. A first core calculates a function by using respective values of the state variable, the intervening variable, and the input variable, and obtains a value of the state variable by time-integration of a value of the function. In the calculation of the function, the value of the state variable calculated in previous time, a value of the intervening variable calculated by the second core in the previous time, and a value of the input variable inputted in this time are used.