A filtration membrane fouling index measuring method, comprising: (a) connecting two same filtration membranes (10a, 10b) in series so as to sequentially pass influent therethrough, wherein fluid which passes through a first filtration membrane of the filtration membranes is then introduced into a second filtration membrane of the filtration membranes; (b) measuring a flow rate (Q I) passing through the first filtration membrane; (c) measuring a flow rate (Q P) passing through the second filtration membrane; (d) measuring a cumulative permeation amount passing through the first or second filtration membrane; and (e) determining a fouling index of each filtration membrane, based on the flow rate passing through the first filtration membrane, the flow rate passing through the second filtration membrane, and the cumulative permeation amount, wherein the fouling index is determined based on β/2 in one of the following Equations: 1 Q I − 1 Q P 2 = β 2 V, where Q I is the flow rate passing through the first filtration membrane, Q P is the flow rate passing through the second filtration membrane, V is the cumulative permeation amount, and β is a cake fouling index reflecting the cake resistance, and: 1 Q OI − 1 Q OP 2 = β 2 V, where Q OI = Q I - Q O, Q OP = Q P - Q O, Q I is the flow rate passing through the first filtration membrane, Q P is the flow rate passing through the second filtration membrane, Q O is a premeasured flow rate when distilled water passes through a filtration membrane having the same membrane properties as those of the filtration membrane, V is the cumulative permeation amount, and β is a cake fouling index reflecting the cake resistance. (a) connecting two same filtration membranes (10a, 10b) in series so as to sequentially pass influent therethrough, wherein fluid which passes through a first filtration membrane of the filtration membranes is then introduced into a second filtration membrane of the filtration membranes; (b) measuring a flow rate (Q I) passing through the first filtration membrane; (c) measuring a flow rate (Q P) passing through the second filtration membrane; (d) measuring a cumulative permeation amount passing through the first or second filtration membrane; and (e) determining a fouling index of each filtration membrane, based on the flow rate passing through the first filtration membrane, the flow rate passing through the second filtration membrane, and the cumulative permeation amount, wherein the fouling index is determined based on β/2 in one of the following Equations: 1 Q I − 1 Q P 2 = β 2 V, where Q I is the flow rate passing through the first filtration membrane, Q P is the flow rate passing through the second filtration membrane, V is the cumulative permeation amount, and β is a cake fouling index reflecting the cake resistance, and: 1 Q OI − 1 Q OP 2 = β 2 V, where Q OI = Q I - Q O, Q OP = Q P - Q O, Q I is the flow rate passing through the first filtration membrane, Q P is the flow rate passing through the second filtration membrane, Q O is a premeasured flow rate when distilled water passes through a filtration membrane having the same membrane properties as those of the filtration membrane, V is the cumulative permeation amount, and β is a cake fouling index reflecting the cake resistance.