PURPOSE: To feed a reduced-pressure gas in a specific pressure range back to part before an adsorption column in a pressure reducing process after adsorption in producing a gas rich in methane from a raw material gas containing CH4 and CO2, by carrying out treatment using a zeolite type adsorbent at ≥a specific temperature.

CONSTITUTION: In pressure reducing process wherein a raw material gas (e.g. liquefied petroleum gas or modified gas from methanol) comprising CH4 as a main component, containing CO2, is passed through an adsorption column packed with an adsorbent of zeolite type having ≥5 average pore diameter at ≥80°C and after adsorption process, the pressure of the column is reduced through atmospheric concentration to vacuum, a reduced-pressure gas in a pressure range of 500±200Torr (preferably 500±100Torr) after starting of pressure reducing operation except a gas to be used for pressure balance is fed back to part before the adsorption column to improve recovery ratio of CH4. The amount of the raw material gas treated can be maintained high and a device can be made compact.