A reactor supplied with a reactant to cause reaction of the reactant includes a reaction device which has a hollow box type member having a top plate and a bottom plate opposed to each other and side plates connected to an edge of the top plate and an edge of the bottom plate. A partition member is housed in the box type member, the partition member coming into contact with at least internal faces of the side plates of the box type member and partitioning a space in the box type member into a plurality of reaction chambers to which the reactant is to be supplied. A penetrating region is provided in the partition member to connect the adjacent reaction chambers to each other, the penetrating region having the reactant passed therethrough.