To treat NOx contained in ventilation gas of a tunnel or the like, since the NOx exerts adverse influence on human bodies and environment, though its content is quite low.

After adsorbing treating object components in gas collected as a part of the atmospheric air to an adsorbent 13, a flow passage is switched to run nitrogen gas with a purity of 90 vol.% or more with an oxygen concentration of 10 vol.% or less to a passage where the adsorbent is present, discharge is generated and non-thermal plasma of nitrogen gas is applied to the adsorbent 13', treating object components are desorbed, the adsorbent is regenerated, and the treating object components are removed by nitrogen plasma in a downstream plasma reactor 14'.