To provide an oligomer capable of giving an excellent color tone and a low content of ether bonds as a by-product to polytrimethylene terephthalate obtained therefrom, and to provide a method for producing the oligomer, excellent in stability of continuous reaction processes.

An ester and/or the oligomer thereof are obtained from a lower alcohol ester of terephthalic acid and 1,3-propanediol, wherein the ester and/or the oligomer satisfy requisites (1) to (5) as follows: main repeating units thereof are composed of trimethylene terephthalate; (2) an average polymerization degree thereof is in the range of 1.5 to 7; (3) cyclic oligomers are contained in a range of 0 to 2.2 wt% based on the ester and/or the oligomer which are obtained from the lower alcohol ester of terephthalic acid and the 1,3-propanediol; (4) hydroxy groups are contained in a range of 92 to 100 mol wt% based on total end groups of the ester and/or the oligomer which are obtained from the lower alcohol ester of terephthalic acid and the 1,3-propanediol; and (5) bis(3-hydroxypropyl)ether is contained in the range of 0-0.3 wt%.