In the present invention, a clogging rate is calculated using a first flow rate function when the degree of clogging of the throttle valve is in the reference state and a second flow rate function estimated based on the intake air volume. For each predetermined period, sample points, which are combinations of the second flow rate function and throttle valve position, are obtained, and the learning points are calculated by averaging a plurality of sample points for each predetermined position range. Based on the plurality of learning points, coefficients a to c of the approximate function of the second flow rate function characteristic are calculated by the least-squares method, and the clogging rate is calculated based on the second flow rate function characteristic and the first flow rate function approximated by the approximate function using the coefficients a to c.