To provide a synthetic resin composition improved in processing stability and heat discoloration resistance.

This composition is prepared by mixing 100 pts.wt. synthetic resin with 0.001-10 pts.wt. phosphorous ester represented by the formula and 0.01-10 pts.wt. another phosphorus-base antioxidant. It is desirable that the antioxidant has a melting point of 40°C or above and that the ratio of the ester compound represented by the formula to the antioxidant is 5:95 to 95:5 by weight.