PURPOSE: To prepare a nontoxic synthetic resin composition having synergistically improved long-term thermal-oxidative stability and excellent processability with suppressing discoloration due to addition of the vitamin, by compounding a resin with phytic acid or its metal salt together with a specific vitamin as an antioxidant.

CONSTITUTION: 100pts.wt. of a synthetic resin is compounded with preferably 0.01W3pts.wt. of a vitamin selected from vitamin B, E and K groups and preferably 0.01W3pts.wt. of phytic acid or its metal salt (Na, K, Ca, etc.). If desired a light stabilizer, pigments, etc. may be added to the composition.