To obtain a polyester-based staple fiber having high hygroscopicity and excellent card passing property by adjusting a moisture regain in a standard condition to ≥ a specific value and a coefficient of static friction to a metal to ≤ a specific value.

This polyester-based staple fiber having ≥3% moisture regain under a standard condition at 20°C and 65% RH is obtained by dipping polyester-based staple fibers in a treating solution containing a monomer contributing hygroscopic property, such as (meth)acrylic acid, a polymerization initiator, a carrier and a pH adjusting agent, subjecting the dipped staple fibers to a graft polymerization process at about 100°C for several ten min to convert a terminal acidic group to an alkali metal salt and treating the product with an oil agent to have a coefficient of static friction to a metal of ≤0.17.