A system includes a particulate matter filter with multiple zones. An abnormal load determination level to be compared with an estimated load obtained by an observer is set for each of the regions. As the drive motor there is used one in which an induced electromotive force is generated when a force in the direction of rotation is applied to the rotary shaft. If the speed of sound in the gas is measured at two states and diluent concentrations other than nitrogen are known, two equations for molecular weight can be equated and solved for the nitrogen concentration in the gas mixture.