To provide a composition which imparts a high viscosity to a polyester elastomer based on a polyester block copolymer containing a large amount of polyalkylene ether glycol, and allows little viscosity change even when exposed to kneading for a long time at a melt kneading step, and a high-viscosity modified polyester elastomer based on the composition.

A polyester elastomer composition contains 100 pts.wt. polyester block copolymer containing 30 to 90 wt.% polyalkylene ether glycol block and 0.1 to 10 pts.wt. oxazoline group-containing copolymer comprising 0.5 to 50 wt.% addition polymerizable oxazoline compound-derived unit and 50 to 99.5 wt.% (meth)acrylic ester-derived unit. In a preparation process of a modified polyester elastic body, this composition is melt kneaded.