It is an object of the present invention to downsize facilities for removing hydrogen sulfide and oxygen in gas and reduce facility cost. Syngas g containing hydrogen sulfide and oxygen as target components for removal or reduction in concentration is contacted with a first material containing transition metal 41 and subsequently with a second material containing transition metal 42 in a first mode. The syngas g is contacted with the second material containing transition metal 42 and subsequently with the first material containing transition metal 41 in a second mode. The first mode and the second mode are alternately executed. In the first mode, iron oxide of the first material containing transition metal 41 reacts with the hydrogen sulfide to become iron sulfide that is reactable with the oxygen and iron sulfide of the second material containing transition metal 42 reacts with the oxygen to become iron oxide that is reactable with the hydrogen sulfide. In the second mode, the iron oxide of the second material containing transition metal 42 reacts with the hydrogen sulfide to become iron sulfide and the iron sulfide of the first material containing transition metal 41 reacts with the oxygen to become iron oxide.