A method of purifying a mixture, the method comprising: feeding a mixture of three or more components to a first distillation column, each of the three or more components having different boiling points; obtaining a lower fraction from a lower portion of the first distillation column, the lower fraction of the first distillation column comprising heavy components, and obtaining an upper fraction from an upper portion of the first distillation column, the upper fraction of the first distillation column comprising light components and a product to be collected; feeding the upper fraction of the first distillation column to a second distillation column, collecting a first lower fraction from a lower portion of the second distillation column, the first lower fraction of the second distillation column comprising the product; and collecting an upper fraction from an upper portion of the second distillation column, the upper fraction of the second distillation column comprising lights components, wherein heat possessed by the upper fraction of the first distillation column is fed to the lower portion of the second distillation column through a first heat exchanger, such that the heat is fed to a second lower fraction collected from the lower portion of the second distillation column, subsequently, feeding a part of the upper fraction of the first distillation column to the second distillation column as a feed stream, and refluxing a remaining part of the upper fraction of the first distillation column to the upper portion of the first distillation column, and condensing and refluxing a part of the light components collected from the upper portion of the second distillation column to the second distillation column, wherein a ratio K of an upper reflux R2 of the second distillation column and an upper reflux R1 of the first distillation column (R2/R1) is 1.05 or more and 1.5 or less; the second lower fraction of the second distillation column to which the heat is fed from the first heat exchanger is refluxed to the second distillation column; and a part of the first lower fraction of the second distillation column is reheated by a reheater and then refluxed to the second distillation column. feeding a mixture of three or more components to a first distillation column, each of the three or more components having different boiling points; obtaining a lower fraction from a lower portion of the first distillation column, the lower fraction of the first distillation column comprising heavy components, and obtaining an upper fraction from an upper portion of the first distillation column, the upper fraction of the first distillation column comprising light components and a product to be collected; feeding the upper fraction of the first distillation column to a second distillation column, collecting a first lower fraction from a lower portion of the second distillation column, the first lower fraction of the second distillation column comprising the product; and collecting an upper fraction from an upper portion of the second distillation column, the upper fraction of the second distillation column comprising lights components, wherein heat possessed by the upper fraction of the first distillation column is fed to the lower portion of the second distillation column through a first heat exchanger, such that the heat is fed to a second lower fraction collected from the lower portion of the second distillation column, subsequently, feeding a part of the upper fraction of the first distillation column to the second distillation column as a feed stream, and refluxing a remaining part of the upper fraction of the first distillation column to the upper portion of the first distillation column, and condensing and refluxing a part of the light components collected from the upper portion of the second distillation column to the second distillation column, wherein a ratio K of an upper reflux R2 of the second distillation column and an upper reflux R1 of the first distillation column (R2/R1) is 1.05 or more and 1.5 or less; the second lower fraction of the second distillation column to which the heat is fed from the first heat exchanger is refluxed to the second distillation column; and a part of the first lower fraction of the second distillation column is reheated by a reheater and then refluxed to the second distillation column.