A filter 2 for collecting PMs, a microwave transmitter 30 for emitting electromagnetic waves whose frequency is dozens of GHz-a few of THz onto the filter 2, a microwave receiver 31 for detecting an intensity of the electromagnetic waves that have transmitted through the filter 2, and a computing means for computing a collection amount of PMs from the intensity that has been detected with the microwave receiver 31 are included. Since it is possible to detect the distribution of the collection amount of PMs with good accuracy, it is possible to carry out a recycling process in a state where the collection amount does not become too much, and thereby it is possible to make a reducing-agent supply amount into exhaust gases minimum at the time of the recycling process.