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.