PROBLEM TO BE SOLVED: To obtain antifouling agent compositions which can exhibit excellent antifouling performance when impregnated into or applied onto base materials which contact seawater or fresh water, such as fishing nets, fishing implements, ships or underwater structures. SOLUTION: Antifouling agent compositions comprise an organopolysiloxane containing an oxyalkylene group to be represented by the formula [wherein R5 is an alkyl group or a phenyl group； R1, R2 and R3 are each independently an alkyl group, a phenyl group or X of the formula: (CH2)pO(C2H4O)q(C3H6O)rR4； m is a number of 1-1,000； and n is a number of 0-50, provided that when n is 0, R1 and/or R2 is X] and having an HLBED based on the ethyleneoxy group (C2H4O) [a total amount (wt.%) of C2H4O in the organopolysiloxane/5] of 1-2.5 and, at the same time, an HLBPO based on the propyleneoxy group (C3H6O) [a total amount (wt.%) of C3H6O in therorganopolysiloxane (wt.%)/5] of 1-10, and excel in stainproofing properties.
