An inner wall 11 of a reactor 10 has a two-layer structure: an anticorrosive layer 11a comprising an alloy material having high anticorrosiveness is provided on the inner side of the reactor contacting a corrosive process gas, and a heat conductive layer 11b for efficiently conducting the heat within the reactor 10 from an inner wall surface to a coolant flow passage 13 is provided on the outer side of the reactor (outer-wall side). The anticorrosive layer 11a comprises an alloy material having a composition for which a value R, defined by R=[Cr]+[Ni]−1.5 [Si], is not less than 40% wherein [Cr] is a mass content (% by mass) of chromium (Cr), [Ni] is a mass content (% by mass) of nickel (Ni), and [Si] is a mass content (% by mass) of silicon (Si).