To provide a compact and economical method for simultaneously achieving the fractional separation and rectification of gas to be treated containing at least two components different in b.p.

Fractional separation is executed in at least two cooling and contact stages. In the first stage, gas is cooled at least partially to condense at least a first liquid fraction and the gas is brought into contact with the liquid fraction at least in a first contact zone Z1 and, at this time, the first liquid fraction is circulated countercurrently with respect to the gas and, at the outlet of the first contact zone Z1, a liquid fraction rich in a component A and a gaseous fraction are individually collected. In the second stage, the gaseous fraction from the stage (a) is treated in the same way as the first stage to individually collect a liquid fraction rich in a component B and a gaseous fraction at the outlet of a second zone.