13 parts by weight comparative example 3 was conducted in the same manner as in example 1 except that the content of the u03b1 methylstyrene acrylonitrile copolymer was changed to 65 parts by weight and that of the styrene acrylonitrile copolymer was changed to 5 parts by weight comparative example 4 was conducted in the same manner as in example 1 except that the content of n substituted maleimide copolymer was changed to 35 parts by weight and that of the a methylstyrene acrylonitrile copolymer was changed to 20 parts by weight comparative example 5 was conducted in the same manner as in example 1 except that n substituted maleimide copolymer was not used and the content of the u03b1 methylstyrene acrylonitrile copolymer was changed to 55 parts by weight as shown in table 2 the composition of comparative example 1 which contained the diene graft polymer less than 15 parts by weight showed a low coefficient of linear thermal expansion but exhibited poor impact strength the composition of comparative example 2 which contained the diene graft polymer more than 30 parts by weight showed high coefficient of linear thermal expansion and good impact strength the composition of comparative example 3 in which the ratio of the u03b1 methylstyrene acrylonitrile copolymer to the styrene.