A stabilizer composition which manifests an improved effect against thermal degradation and oxidation degradation of an organic material comprises a phosphite of formula (I)
wherein R 1 , R 2 , R 4 and R 5 each independently represent a hydrogen atom, an alkyl group having 1 to 8 carbon atoms, a cycloalkyl group having 5 to 8 carbon atoms, an alkylcycloalkyl group having 6 to 12 carbon atoms, an aralkyl group having 7 to 12 carbon atoms or a phenyl group, R 3 and R 6 each independently represent a hydrogen atom or an alkyl group having 1 to 8 carbon atoms, X 1 represents a divalent alcohol residue, X 2 represents a direct bond or an alkylene group having 1 to 8 carbon atoms, and Y represents a hydroxyl group, and an amine (A).