Polyester resin composition (C) comprises polyester resin (A), which comprises polyester resin (l) comprising a diol unit having a cyclic acetal structure (CA) and a dicarboxylic acid unit having no CA, polyester resin (m) comprising a dicarboxylic acid unit having CA and a diol unit having no CA and/or polyester resin (n) comprising a diol unit having CA and a dicarboxylic acid unit having CA, and polyester resin (B) having no CA. The sum of the fraction of the dicarboxylic acid unit having CA in the entire dicarboxylic acid unit and the fraction of the diol unit having CA in the entire diol unit in composition (C) is 0.05% by mole or greater. The polyester resin compositions are excellent in transparency, melt viscoelasticity, heat resistance and moldability, and exhibit a samll crystallization rate.