PURPOSE: To prevent a filter from being clogged with impurities and thereby reduce the frequency of the filter replacement by cleaning the inner surface of a fibrous cylindrical filter element of a cartridge type filter with the gas in a solution sent pressurized thereinto.

CONSTITUTION: A solution 1 is sampled from a solution tank 4 through a sampling pipe 7 and sent pressurized by a diaphragm pump 5 into a fibrous cylindrical filter element 9 set in a cylindrical case 12 of a cartridge type filter 6, thereby obtaining the solution filtered through the filter element 9 from the cylindrical case 12 for concentration analysis. At this time, the solution is sampled from the solution tank 4 through the sampling pipe 7, while simultaneously drawing air therein, and a mixture of the solution and the air is sent pressurized by the diaphragm pump 5 into the fibrous cylindrical filter element 9 set in the cylindrical case 12 of the filter 6, thereby cleaning the inner surface of the filter element 9 with the gas in the solution 1, resulting in a substantial reduction in the frequency of the filter replacement.