The present disclosure relates to an apparatus for preparing an oligomer, the apparatus including: a reactor for oligomerizing a feed stream containing a fed monomer; a stirrer inserted into a hole formed in an upper portion of the reactor; and a solvent transfer line extending inward from a side of the reactor, wherein the stirrer includes a rotating shaft vertically extending downward from the upper portion of the reactor, and a blade having a conical shape whose vertex is positioned at a lower end of the rotating shaft and outer diameter increases from a bottom toward a top, and the solvent transfer line has a plurality of spray nozzles formed in a direction toward the blade.