Treatment agent for elastic fibers, and elastic fiber

An elastic fiber treatment agent comprising a smoothing agent and an alkyl phosphate ester compound, wherein

• the alkyl phosphate ester compound contains a phosphate ester Q1 represented by the following formula (1) and at least one or more selected from the group consisting of a phosphate ester Q2 represented by the following formula (2) and a phosphate ester Q3 represented by the following formula (3),
• at least one of M1 to M4 in the following formulas (1) to (3) is an alkaline earth metal, and
• in a P nucleus NMR measurement of the alkyl phosphate ester compound upon an alkali over-neutralization pretreatment, a P nucleus NMR integral ratio attributed to the phosphate ester Q1 is 15% to 60% if the sum of P nucleus NMR integral ratios attributed to the phosphate ester Q1, the phosphate ester Q2, the phosphate ester Q3, and phosphoric acid and its salt is taken as 100%,
• In the formula (1),
• R1 is an alkyl group with 4 to 24 carbon atoms, and
• M1 and M2 are each a hydrogen atom, an alkali metal, an alkaline earth metal, ammonium, phosphonium, or an organic amine salt;
• In the formula (2),
• R2 and R3 are each an alkyl group with 4 to 24 carbon atoms, and
• M3 is a hydrogen atom, an alkali metal, an alkaline earth metal, ammonium, phosphonium, or an organic amine salt;
• In the formula (3),
• R4 and R5 are each an alkyl group with 4 to 24 carbon atoms,
• n is an integer of 2 or more, and
• M4 is a hydrogen atom, an alkali metal, an alkaline earth metal, ammonium, phosphonium, or an organic amine salt, provided that when the number of M4 present in the molecule is 2 or more, they may be identical with or different from each other.