The present invention provides an antifouling ability-imparting agent that adheres well to a variety of water treatment membranes such as reverse osmosis membranes and imparts excellent antifouling ability to the water treatment membranes. The present invention relates to an antifouling ability-imparting agent containing a copolymer, the copolymer containing: a structural unit (I) represented by the following formula (1):




wherein R1 is a hydrogen atom or a methyl group; R2 is a direct bond, —CH2—, —CH2CH2—, or —CO—; R3s are the same as or different from each other and are each a C1-C20 alkylene group; X is —CH2CH(OH)CH2(OH) or —CH(—CH2OH)2; and n is a number of moles of oxyalkylene groups added and is 0 to 100; and a structural unit (II) represented by the following formula (2):





wherein R4 is a hydrogen atom or a methyl group; R5 is a direct bond, —CH2—, —CH2CH2—, or —CO—; R6s are the same as or different from each other and are each a C1-C20 alkylene group; R7 is a hydrogen atom or a C1-C20 alkyl group; and m is a number of moles of oxyalkylene groups added and is 1 to 100.