An estimated quantity of soot deposited near a movable part of a variable capacity mechanism and an estimated quantity of soot burnt off therefrom are determined in advance in accordance with values of parameters (engine rotation speed or similar) that express the engine operating condition, and stored in storage means. Then actual values of the parameter are measured, and an estimated deposition quantity Sa and an estimated burn off quantity Sb corresponding to the measured values are determined from the storage means. An estimated remaining quantity of soot Kn is determined by obtaining the difference between the estimated deposition quantity and the estimated burn off quantity, and a movable vane is forcibly driven to open and close in accordance with the estimated remaining quantity Kn.