To provide polycondensation copolymers which give molded articles excellent in gas barrier properties, particularly prevention of the change in the properties of the contents by oxygen, transparency and heat resistance and not only low in aldehyde content but also excellent in bottle strength and hard to cause laminar peeling even if a knife is run into a bottle.

A polycondensation copolymer comprises [I] a polyester component composed of (i) a dicarboxylic acid constituting unit having 5-100 mol%, based on the entire dicarboxylic acid constituting units, constituting units derived from terephthalic acid and 0-95 mol% constituting units derived from isophthalic acid and (ii) a diol constituting unit having 50-100 mol%, based on the entire diol constituting units, constituting units derived from ethylene glycol and 0-50 mol% constituting units derived from 1,3-bis(2-hydroxyethoxy)benzene and [II] a polyolefin oligomer component, and has an intrinsic viscosity [η] of 0.5-1.5 dl/g. A polycondensation copolymer molded product has at least one layer of the polycondensation copolymer.