To provide an operation method of a distillation tower by which the distillation tower is stably operated especially in the operation state wherein maximum capacity is exhibited while saving the power.

The operation method is a method for a distillation tower as to keep a gas amount in the distillation tower at one value among a plurality of gas amount level values previously determined by a programmed computer and comprises the following respective means. 1: a means of collecting data of the operation state of the distillation tower; 2: a means of recording the collected data of the operation state of the distillation tower as time series data; 3: a means of computing the optimum gas amount level value from the recorded data of the operation state of the distillation tower and the data of separation conditions in the distillation tower; 4: a means of computing the optimum gas amount level value based on the computed optimum gas amount and the amount at the time; 5: a means of changing the operation of the distillation tower so as to adjust the gas amount to be the optimum gas amount level value in the case the optimum gas amount level value and the gas amount level value at the time are different; and 6: a means of driving the distillation tower as to keep the gas amount to be the optimum gas amount level value.