PURPOSE: To rapidly and accurately detect the breakage of a filter cloth regardless of its degree by arranging an automatic fine particle measuring device in a flue extending from the outlet of a dust collecting chamber to a stack to continuously analyze the solid content of a treated gas and detecting the breakage of the filter cloth from the change in the content.

CONSTITUTION: A waste gas A is introduced into a dust collecting chamber, and the solid is removed by a filter cloth 2 to obtain the treated gas B that is discharged into the atmosphere from a stack 6. An automatic fine particle measuring device 5 is arranged in a flue 3 extending from the outlet of the dust collecting chamber to the stack 6 to detect the breakage of the filter cloth 2 used in the bag filter 1. The solid content of the treated gas B is continuously analyzed by the device, and the breakage of the filter cloth 2 is detected from the change in the content. Consequently, the breakage of the filter cloth is rapidly and accurately detected regardless of its degree.