an agent and method for treating biodegradable synthetic yarns fabricated from a polymer comprising lactic acid as a main component , which enable improved lubricity , cohesion , etc . to be so imparted to the biodegradable synthetic yarns that the yarns can be prevented from fuzzing and breaking at every step from spinning to down - stream step , especially at a false twisting step and improved in terms of bulkiness , providing yarns having improved mechanical properties in a stable manner . the agent of the invention comprises 0 . 1 to 30 weight % of a specific functional agent , and a lubricant and a surfactant in the total amount of 70 weight % or greater , and has a friction coefficient in the range of 0 . 04 to 0 . 35 .