A device for measuring the pollution index of a filtration membrane, the device comprising: a raw water supply part supplying raw water to be measured; a plurality of filtration membranes (110, 120, 130) having different pore sizes, wherein the plurality of filtration membranes comprises at least a first filtration membrane, a second filtration membrane and a third filtration membrane, the pore sizes of the first, second, and third filtration membrane (110, 120, and 130) are ranked in descending order; a plurality of path lines (21a, 22a, 23a) connecting the plurality of filtration membranes (110, 120, 130) in parallel to the raw water supply part; a plurality of flow measuring parts (310a, 310b, 310c) installed on the outlet sides of the filtration membranes (110, 120, 130) to correspond to the respective filtration membranes (110, 120, 130), and measuring the flow rate of raw water passed through the filtration membranes (110, 120, 130); and a pollution index measuring part measuring the pollution indexes of the filtration membranes (110, 120, 130) on the basis of the flow rates measured by the flow rate measuring parts (310a, 310b, 310c); characterized in that the device further comprises: at least one auxiliary filtration membrane (140a,140b) installed on the inlet side of at least one of the plurality of filtration membranes (110, 120, 130), wherein the at least one auxiliary filtration membrane (140a,140b) comprises a first auxiliary filtration membrane installed at the inlet of the second filtration membrane and having a larger pore size than that of the second filtration membrane, and a second auxiliary filtration membrane installed at the inlet of the third filtration membrane and having a larger pore size than that of the third filtration membrane. a raw water supply part supplying raw water to be measured; a plurality of filtration membranes (110, 120, 130) having different pore sizes, wherein the plurality of filtration membranes comprises at least a first filtration membrane, a second filtration membrane and a third filtration membrane, the pore sizes of the first, second, and third filtration membrane (110, 120, and 130) are ranked in descending order; a plurality of path lines (21a, 22a, 23a) connecting the plurality of filtration membranes (110, 120, 130) in parallel to the raw water supply part; a plurality of flow measuring parts (310a, 310b, 310c) installed on the outlet sides of the filtration membranes (110, 120, 130) to correspond to the respective filtration membranes (110, 120, 130), and measuring the flow rate of raw water passed through the filtration membranes (110, 120, 130); and a pollution index measuring part measuring the pollution indexes of the filtration membranes (110, 120, 130) on the basis of the flow rates measured by the flow rate measuring parts (310a, 310b, 310c); characterized in that the device further comprises: at least one auxiliary filtration membrane (140a,140b) installed on the inlet side of at least one of the plurality of filtration membranes (110, 120, 130), wherein the at least one auxiliary filtration membrane (140a,140b) comprises a first auxiliary filtration membrane installed at the inlet of the second filtration membrane and having a larger pore size than that of the second filtration membrane, and a second auxiliary filtration membrane installed at the inlet of the third filtration membrane and having a larger pore size than that of the third filtration membrane.