It is used a filter trapping particulates from a gas containing the particulates, a container 5 containing the filter, an upstream pipe 3 provided on the upstream side of the container 5 to lead the gas "A" into the container 5, a downstream pipe 4 provided on the downstream side of the container 5 to lead the gas "B" after the gas passed through the filter, a transmitting antenna 11 to transmit an electromagnetic wave having a frequency of 80 GHz or more and 200 GHz or less, and a receiving antenna 10 to receive the electromagnetic wave. An amount of the particulates trapped in the filter is detected based on an intensity of the electromagnetic wave received by the receiving antenna 10.