An operating method of a distillation column includes: adjusting a fluid being treated, which is supplied to the distillation column, to a turndown-operation-time supply flow rate that is lower than a design-operation supply flow rate; increasing a turndown flux flow rate and a turndown reboiler flow rate, and raising a column-internal air-liquid load, in order to make the properties of a column-bottom product flowing out from the column bottom heavier than those in the design operation while satisfying specifications required in design, or making the properties of a column-top product flowing out from the column top heavier than those in the design operation while satisfying specifications required in design, for the distillation column to which the fluid being treated is supplied at the turndown-operation-time supply flow rate; and verifying that the properties of the column-top product or the column-bottom product satisfy required specifications.