An apparatus and a method for separating a specified gas from a gas to be treated containing the specified gas comprising at least one ingredient, which comprises allowing the gas to be treated to flow through a column without the use of another gas for transferring the gas to be treated, while keeping the inside of the column packed with a packing material at a reduced pressure. The above apparatus and method can be suitably used for separating a specified gas having a high purity at a low cost.