A method of using a distillation device, comprising a first distillation unit that comprises a first condenser, a first reboiler and a first distillation column; a second distillation unit that comprises a second condenser, a second reboiler, and a second distillation column; and a heat exchanger, wherein a raw material comprising a compound represented by Formula 1 below and an isomer of the compound is introduced into a first supply port of the first distillation column and/or a second supply port of the second distillation column, the raw material introduced into the first supply port of the first distillation column is separately discharged into each of a first column top stream discharged from an upper section of the first distillation column; and first, second and third column bottom streams separately discharged from a lower section of the first distillation column, the first column top stream is introduced into the first condenser, and a portion or all of the first column top stream via the first condenser is refluxed in the upper section of the first distillation column, the first column bottom stream is introduced into the first reboiler, and the first column bottom stream via the first reboiler is refluxed in the lower section of the first distillation column, the stream introduced into the second supply port of the second distillation column is separately discharged to each of a second column top stream discharged from an upper section of the second distillation column; and fourth and fifth column bottom streams discharged from a lower section of the second distillation column, the fourth column bottom stream is introduced into the second reboiler, and the fourth column bottom stream via the second reboiler is refluxed in the lower section of the second distillation column, the third column bottom stream and the second column top stream are introduced into the heat exchanger and exchange heat therebetween, the third column bottom stream via the heat exchanger is refluxed in the lower section of the first distillation column, the second column top stream via the heat exchanger is introduced into the second condenser, and a portion or all of the second column top stream via the second condenser is refluxed in the upper section of the second distillation column, and Equations 1 and 2 are satisfied, wherein the compound represented by Formula 1 is n-butyl aldehyde, and the isomer of the compound is iso-butyl aldehyde: wherein R is a C 1 to C 12 alkyl group; T t − 2 − T b − 3 ≥ 8 ° C P 2 / P 1 ≥ 20 wherein T t-2 indicates a temperature of the second column top stream, and T b-3 indicates a temperature of the third column bottom stream, and P 1 indicates a pressure of the upper section of the first distillation column (kg/cm 2 g), and P 2 indicates a pressure of the upper section of the second distillation column (kg/cm 2 g). wherein T t-2 indicates a temperature of the second column top stream, and T b-3 indicates a temperature of the third column bottom stream, and P 1 indicates a pressure of the upper section of the first distillation column (kg/cm 2 g), and P 2 indicates a pressure of the upper section of the second distillation column (kg/cm 2 g). <EOS>