An apparatus for preparing oligomer, comprising: a reactor receiving a monomer stream and a solvent stream, allowing an oligomerization reaction to proceed, and discharging a gaseous first discharge stream and a liquid second discharge stream; a gas-liquid separator separating the gaseous first discharge stream of the reactor into a liquid phase and a gas phase and feeding a liquid bottom discharge stream to the reactor; a first transfer line extending from a bottom of the gas-liquid separator to a region spaced apart from an inner wall of the reactor at a position higher than a reaction region inside the reactor and transferring the liquid bottom discharge stream of the gas-liquid separator; a solvent transfer line transferring the solvent stream to the reactor; a second transfer line separated from the solvent transfer line and transferring a part of the solvent stream to a region spaced apart from the inner wall of the reactor at a position higher than a height at which the first transfer line inside the reactor is formed; a first spray nozzle unit connected to an end of the first transfer line inside the reactor and having n spray nozzles formed to be spaced apart from each other, wherein n is an integer of 2 or more; and a second spray nozzle unit connected to an end of the second transfer line inside the reactor and having n-1 spray nozzles formed to be spaced apart from each other, wherein n is an integer of 2 or more, wherein a spray angle of the liquid bottom discharge stream of the gas-liquid separator sprayed from the spray nozzles formed in the first spray nozzle unit is 30° to 90°, and a spray angle of the solvent stream sprayed from the spray nozzles formed in the second spray nozzle unit is 91° to 120°. a reactor receiving a monomer stream and a solvent stream, allowing an oligomerization reaction to proceed, and discharging a gaseous first discharge stream and a liquid second discharge stream; a gas-liquid separator separating the gaseous first discharge stream of the reactor into a liquid phase and a gas phase and feeding a liquid bottom discharge stream to the reactor; a first transfer line extending from a bottom of the gas-liquid separator to a region spaced apart from an inner wall of the reactor at a position higher than a reaction region inside the reactor and transferring the liquid bottom discharge stream of the gas-liquid separator; a solvent transfer line transferring the solvent stream to the reactor; a second transfer line separated from the solvent transfer line and transferring a part of the solvent stream to a region spaced apart from the inner wall of the reactor at a position higher than a height at which the first transfer line inside the reactor is formed; a first spray nozzle unit connected to an end of the first transfer line inside the reactor and having n spray nozzles formed to be spaced apart from each other, wherein n is an integer of 2 or more; and a second spray nozzle unit connected to an end of the second transfer line inside the reactor and having n-1 spray nozzles formed to be spaced apart from each other, wherein n is an integer of 2 or more, wherein a spray angle of the liquid bottom discharge stream of the gas-liquid separator sprayed from the spray nozzles formed in the first spray nozzle unit is 30° to 90°, and a spray angle of the solvent stream sprayed from the spray nozzles formed in the second spray nozzle unit is 91° to 120°.