To provide the subject device simple in a structure, inexpensive, capable of precisely detecting the clogging from the outside and applicable even to an end pipe.

In this device, the bypass line of a filter device 1 such as a strainer 2 is constituted of a pressure guiding pipe 4 where a diaphragm 7 is interposed and a lead 8 is adhered along the diaphragm 7 so that the diaphragm 7 is deformed when a pressure drop due to the clogging of the strainer 2 becomes large and the lead 8 is broken at a prescribed deformed amount of the diaphragm as the result in a prescribed pressure drop. Buffers 5 are interposed on the bypass line so that the lead 8 is not broken carelessly by a water hammer, and the like, in order to detect only the clogging state of the filter device 1 by the break itself.