An air separation device for distilling air at a low temperature, includes a high-pressure column to separate high-pressure raw material air into high-pressure nitrogen gas and high-pressure oxygen-enriched liquefied air; a low-pressure column to separate the high-pressure oxygen-enriched liquefied air into low-pressure nitrogen gas, low-pressure liquefied oxygen, and argon-enriched liquefied oxygen; an argon column to separate the argon-enriched liquefied oxygen having a pressure higher than the pressure into argon gas and medium-pressure liquefied oxygen; first and second indirect heat-exchangers; first and second gas-liquid separation chambers; a first/second passage which communicates the gas/liquid phase of the low-pressure column and the gas phase of the second gas-liquid separation chamber; and a first/second opening/closing mechanism located on the first/second passage.