Abstract: Disclosed is a method for stabilizing a carbodiimide derivative (1) which is a condensing agent useful for production of general synthetic chemical products. Also disclosed is a stabilized composition of the carbodiimide derivative. The method is characterized in that the carbodiimide derivative (1) is handled in an atmosphere wherein the concentration of molecular oxygen in the gas phase within a container is set at not more than 3% by volume and/or in the co-presence of at least one compound selected from the group consisting of antioxidants, N-oxyl compounds, sulfur compounds, amines and Lewis acids.