A polyester resin composition includes: a polylactic acid; a poly(3-hydroxyalkanoate); pentaerythritol; and a silicate. In the polyester resin composition, an amount of the polylactic acid is from 55 to 75 parts by weight, and an amount of the poly(3-hydroxyalkanoate) (B) is from 25 to 45 parts by weight, with respect to 100 parts by weight of a total amount of the polylactic acid and the poly(3-hydroxyalkanoate); an amount of the pentaerythritol is from 0.05 to 20 parts by weight with respect to 100 parts by weight of the total amount of the polylactic acid and the poly(3-hydroxyalkanoate); and an amount of the silicate is from 10 to 40 parts by weight with respect to 100 parts by weight of the total amount of the polylactic acid and the poly(3-hydroxyalkanoate).