Disclosed is an elastic fiber treatment agent that contains a smoothing agent and an alkyl phosphate ester compound. The alkyl phosphate ester compound contains specific phosphate ester Q1 and at least one or more selected from specific phosphate esters Q2 and Q3. 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 esters Q1 to Q3 and phosphoric acid and its salt is taken as 100%.

Disclosed is an elastic fiber treatment agent that contains a smoothing agent and an alkyl phosphate ester compound. The alkyl phosphate ester compound contains specific phosphate ester Q1 and at least one or more selected from specific phosphate esters Q2 and Q3. 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 esters Q1 to Q3 and phosphoric acid and its salt is taken as 100%.

Disclosed is an elastic fiber treatment agent that contains a smoothing agent and an alkyl phosphate ester compound. The alkyl phosphate ester compound contains specific phosphate ester Q1 and at least one or more selected from specific phosphate esters Q2 and Q3. 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 esters Q1 to Q3 and phosphoric acid and its salt is taken as 100%.

Disclosed is an elastic fiber treatment agent that contains a smoothing agent and an alkyl phosphate ester compound. The alkyl phosphate ester compound contains specific phosphate ester Q1 and at least one or more selected from specific phosphate esters Q2 and Q3. 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 esters Q1 to Q3 and phosphoric acid and its salt is taken as 100%.