An object of the present invention is to provide a modifier which improves moldability in melt molding, particularly, injection molding, extrusion molding, profile molding, direct blow molding, or calendar processing molding using a polyester resin and, furthermore, which can improve mechanical physical property while maintaining transparency, and a polyester molded article using the modifier. The present invention relates to a modifier for a polyester resin, comprising an amorphous polyester resin (I), and a reactive compound (II) containing two or more glycidyl groups and/or isocyanate groups per one molecule and having a weight average molecular weight of not less than 200 and not more than 500 thousands.