Provided is a resin acid ester-based tackifier having a good compatibility with a synthetic rubber elastomer, being capable of reducing the melt viscosity of the elastomer, and being capable of imparting excellent adhesive properties and/or pressure-sensitive adhesive properties to the elastomer. The tackifier is composed of an ester composition characterized by
(i) the ester composition comprising 70% by weight or more of a resin acid ester represented by the general formula (1):
and
(ii) the S′NMR/SNMR ratio of the ester composition being at least 6% in the 1H-NMR spectrum of the ester composition, wherein S′NMR is the total integral value of the proton signal(s) appearing in the region of 6 to 8 ppm and SNMR is the total integral value of all the proton signals appearing over the entire range of the 1H-NMR spectrum.