To solve the problem that clogging of a conduit in all modes of high-pressure and/or low-pressure sides is not accurately detected when the clogging of the conduit is detected based on the magnitude of calculated variance value and correlation coefficient of a differential pressure signal and a static pressure signal.

The variance of the differential pressure signal is compared to its normal value to detect the clogging of the conduit. Which side of the conduit the clogging is present is detected from a variance ratio of the differential pressure signal and the static pressure signal, or a ratio between the correlation coefficient of the differential and static pressure signals and the autocorrelation function of the static pressure signal. Thus, the clogging of the conduit in all modes can be accurately detected.