A gas decomposition filter unit (10A) of the present invention includes: a housing (1A) including gas decomposition pellets (3) and having a mesh (5) as a side surface vertical to a direction in which gas flows, which mesh (5) having a mesh size smaller than each of the gas decomposition pellets (3) and a light guide member 4A for causing light having entered from a light source (2) to exit toward the gas decomposition pellets (3), which light guide member 4A is provided along at least one direction so as to extend over a spatial range in which the plurality of gas decomposition pellets (3) are provided along the side surface of the housing.
