A distillation apparatus for NMP in which a spent NMP comprising light-boiling components and high-boiling components as impurities is purified, said distillation apparatus comprising a raw material tank for storing the spent NMP as a liquid to be treated, a distillation column in which the liquid to be treated which is supplied from the raw material tank is subjected to distillation to obtain a purified high-purity NMP, and a product tank for storing the high-purity NMP obtained in the distillation column, in which: the distillation column is a side-cut type distillation column that comprises a top portion in which the liquid to be treated is separated into a high-concentration NMP having a concentration of not less than 99% by weight and water comprising the light-boiling components, a bottom portion in which the liquid to be treated is separated into the high-purity NMP having a concentration of not less than 99.9% by weight and the high-concentration NMP comprising the high-boiling components, and a mid-stage portion from which the high-purity NMP is withdrawn as a side-cut vapor; and the distillation column is further provided at a rear stage thereof, with a condenser for condensing the high-purity NMP withdrawn as the side-cut vapor, a flow path for withdrawing the side-cut vapor which extends up to the condenser is connected to the mid-stage portion of the distillation column, and an inner circumferential portion of the distillation column is equipped with a liquid guard cover of an open-bottomed box shape at a connecting position thereof to the flow path for withdrawing the side-cut vapor; and a flow control valve as a flow control means for regulating an amount of a liquid of the high-purity NMP withdrawn from the condenser to the product tank on the basis of a height of a liquid level in the bottom portion of the distillation column so as to control and maintain the height of the liquid level in the bottom portion of the distillation column in a predetermined range. the distillation column is a side-cut type distillation column that comprises a top portion in which the liquid to be treated is separated into a high-concentration NMP having a concentration of not less than 99% by weight and water comprising the light-boiling components, a bottom portion in which the liquid to be treated is separated into the high-purity NMP having a concentration of not less than 99.9% by weight and the high-concentration NMP comprising the high-boiling components, and a mid-stage portion from which the high-purity NMP is withdrawn as a side-cut vapor; and the distillation column is further provided at a rear stage thereof, with a condenser for condensing the high-purity NMP withdrawn as the side-cut vapor, a flow path for withdrawing the side-cut vapor which extends up to the condenser is connected to the mid-stage portion of the distillation column, and an inner circumferential portion of the distillation column is equipped with a liquid guard cover of an open-bottomed box shape at a connecting position thereof to the flow path for withdrawing the side-cut vapor; and a flow control valve as a flow control means for regulating an amount of a liquid of the high-purity NMP withdrawn from the condenser to the product tank on the basis of a height of a liquid level in the bottom portion of the distillation column so as to control and maintain the height of the liquid level in the bottom portion of the distillation column in a predetermined range.