PURPOSE: To automatically and accurately detect the clogging of a filter, by detecting the clogging of the filter from the difference of the inlet pressure and the outlet pressure of a fluid passing the filter.

CONSTITUTION: The inlet pressure 4 and the outlet pressure 5 of a fluid passing a filter 2 required in order to carry out the function of installation 1 requiring the fluid are inputted to an operation apparatus 6 at constant cycles and the differential pressure P thereof is operated. In this case, it is operated whether the differential pressure ΔP exceeds tolerant differential pressure ΔP1 being an objective value or not and, when exceeds the objective value ΔP1, an alarm order is issued to an alarm apparatus 7. By this alarm, the filter 2 is cleaned or replaced to keep the capacity of the filter 2 within a definite standard. As the result, an adequate filter cleaning time can be known and the lowering in the efficiency of the installation 1 or wasteful energy consumption can be prevented.