A distillation apparatus of the present invention includes a distillation column group in which a plurality of distillation columns comprising a condenser and a reboiler is connected in the form of a cascade; a gas-feeding line (11) which feeds gas from the distillation column to a latter distillation column (12); a gas line (13) which introduces the gas from the distillation column (12) to the condenser (14) attached to the distillation column (12); a liquid-line which withdraws a condensed liquid from the condenser (14); a liquid-reflux line (18) which introduces a part of the condensed liquid from the liquid-line to the distillation column (12); a liquid-return line (17) which returns the remainder of the condensed liquid from the liquid-line to a former distillation column; a valve (19) provided on the liquid-return line; and a bypass line (20) which connects the liquid-return line and the gas line (13) so as to flow an evaporated gas generated in the liquid-return line (17) to the gas line (13).
