A distillation apparatus for NMP in which a spent NMP comprising low-boiling components and high-boiling components as impurities is purified, which distillation apparatus comprises: a raw material tank for storing the spent NMP as a liquid to be treated; a first distillation column which is constructed and arranged such that, in operation, the liquid to be treated, which is supplied from the raw material tank, is subjected to distillation and separated into a high-concentration NMP having a concentration of not less than 99% and water comprising the low-boiling components as bottoms and a distillate, respectively; a second distillation column which is constructed and arranged such that, in operation, the bottoms from the first distillation column are further subjected to distillation and separated into a high-purity NMP having a concentration of not less than 99.9% and the high-concentration NMP comprising the high-boiling components as a distillate and bottoms, respectively; and a product tank for storing the high-purity NMP obtained as the distillate from the second distillation column, and wherein the distillation apparatus is provided with a control device for controlling the first distillation column and the second distillation column by executing an automatic treatment function, the automatic treatment function comprising: a start-up function for performing, after previously performing a reduced pressure operation of each of the first and second distillation columns (1, 2), a circulation operation in which the liquid to be treated is fed from the raw material tank to the first distillation column, the distillate from the second distillation column is fed back to the raw material tank, and the distillate from the first distillation column is fed back to the raw material tank, to control the respective distillation columns (1, 2) in a stationary state, and then initiating a continuous treatment operation of the distillation apparatus; and an operational mode switching function for switching, upon the continuous treatment operation, an operational mode of the distillation apparatus again to the circulation operation in the case where a level of the liquid in the raw material tank is lowered to a predetermined height during the continuous treatment operation or in the case where a level of the liquid in the product tank is raised to a predetermined height during the continuous treatment operation. a raw material tank for storing the spent NMP as a liquid to be treated; a first distillation column which is constructed and arranged such that, in operation, the liquid to be treated, which is supplied from the raw material tank, is subjected to distillation and separated into a high-concentration NMP having a concentration of not less than 99% and water comprising the low-boiling components as bottoms and a distillate, respectively; a second distillation column which is constructed and arranged such that, in operation, the bottoms from the first distillation column are further subjected to distillation and separated into a high-purity NMP having a concentration of not less than 99.9% and the high-concentration NMP comprising the high-boiling components as a distillate and bottoms, respectively; and a product tank for storing the high-purity NMP obtained as the distillate from the second distillation column, and wherein the distillation apparatus is provided with a control device for controlling the first distillation column and the second distillation column by executing an automatic treatment function, the automatic treatment function comprising: a start-up function for performing, after previously performing a reduced pressure operation of each of the first and second distillation columns (1, 2), a circulation operation in which the liquid to be treated is fed from the raw material tank to the first distillation column, the distillate from the second distillation column is fed back to the raw material tank, and the distillate from the first distillation column is fed back to the raw material tank, to control the respective distillation columns (1, 2) in a stationary state, and then initiating a continuous treatment operation of the distillation apparatus; and an operational mode switching function for switching, upon the continuous treatment operation, an operational mode of the distillation apparatus again to the circulation operation in the case where a level of the liquid in the raw material tank is lowered to a predetermined height during the continuous treatment operation or in the case where a level of the liquid in the product tank is raised to a predetermined height during the continuous treatment operation.