A catalytic reactor confining a catalyst in a mid-portion of a tubular housing made up of a pair of shells flanged sideways and welded together. The cross-sectional area of the housing becomes largest in the mid-portion, and the assembled shells define therein a fore-portion diverging towards the mid-portion and an aft-portion converging from the mid-portion. Side flanges of each shell are formed substantially over the entire length of the shell, and the width of the flanges is enlarged in both the fore- and aft-portions. The housing has two coupling flanges welded respectively at the fore- and aft-ends of the housing both to the outer periphery of the shells and to the ends of the side flanges. When the reactor confines therein a porous monolithic core shaped similarly to the mid-portion of the housing and treated with a catalyst, a buffer layer of wire mesh covers the periphery of the core so as to be tightly interposed between the core and the inner surfaces of the assembled shells, and marginal regions of the buffer layer are made to protrude beyond the end faces of the core so as to be forcibly folded inwards during assemblage of the reactor by two peripheral shoulders formed on the inside of the shells at fore- and aft-boundaries of the mid-portion.