1. An intake parameter-calculating device for an internal combustion engine, for calculating an intake parameter indicative of a state of air in an intake passage in the engine in which an amount of air passing through an intake throttle valve is changed, as a passing air amount, by the intake throttle valve disposed in the intake passage, comprising: basic intake parameter-calculating means for calculating a basic intake parameter as a basic value of the intake parameter; first opening function value-calculating means for calculating a first opening function value as a first calculated value of an opening function value, using a model equation derived by a predetermined modeling method and defining a relationship between an upstream-side pressure which is a pressure in the intake passage on an upstream side of the intake throttle valve, a downstream-side pressure which is a pressure in the intake passage on a downstream side of the intake throttle valve, the opening function value determined by a degree of opening of the intake throttle valve, and the passing air amount; second opening function value-calculating means for calculating a second opening function value as a second calculated value of the opening function value, using a correlation model representative of a correlation between the degree of opening of the intake throttle valve and the opening function value; correction value-calculating means for calculating a correction value using a function value ratio which is a ratio between one and the other of the calculated first opening function value and the calculated second opening function value; and intake parameter-calculating means for calculating the intake parameter by correcting the basic intake parameter using the calculated correction value.