Abstract: The present invention relates to a method and a device for purifying methane gas in a Raman cell (20). When pumped laser light having a high power passes through the Raman cell containing methane gas, a portion of the methane can be decomposed, and then highly reactive methyl radicals are formed. They can react with each other and as time passes form organic compounds, which are deposited on the inner side of the windows (22a, 22b) of the Raman cell. A trap (26) comprising calcium grains (27), which absorb the methyl radicals, is introduced in the Raman cell (20). The methane gas, which comprises a small content of methyl radicals, is made to come in contact with the calcium grains (27), whereby a reaction occurs between the methyl radicals and the calcium grains, so that calcium carbide and calcium hydride are formed. The methane gas is driven around in the Raman cell (20) and through the trap (26) by means of a compressor wheel (28) driven by a motor (29c).