Softening agent

A method for treating fibers comprising bringing a treatment liquid into contact with the fibers, the treatment liquid comprising a softening agent and water, the softening agent comprising:

• a component (A) which is sodium di(2-propylheptyl) sulfosuccinate;
• and
• a component (B) which is at least one alcohol selected from the 1-heptanol, 1-octanol, 1-nonanol, 1-decanol, 1-undecanol, 1-dodecanol, 1-tridecanol, and 1-tetradecanol,
• wherein a proportion of the component (A) relative to a total of 100 parts by mass of the components (A) and (B) is 50 parts by mass to 90 parts by mass, and a proportion of the component (B) relative to the total of 100 parts by mass of the components (A) and (B) is 10 parts by mass to 50 parts by mass
• wherein R1 and R2 each independently represent a hydrocarbon group with 6 or more and 24 or less carbons, with the proviso that at least one of R1 and R2 is a hydrocarbon group having a branch chain, A1O and A2O each independently represent an alkyleneoxy group with 2 or more and 4 or less carbons, x1 and x2 each independently represent an average number of added moles, which is a number of 0 or more and 10 or less, and M is a cation.