The present invention relates to a reactor 8 which is installed in a gas collection chamber 7A (flow change part) through which exhaust gas 1 of an exhaust system flows in a meandering manner and which is configured so as to be impinged on by a spray of urea water 10 (additive) sprayed from the upstream side of the reactor and receive heat therefrom so as to promote vaporization of the spray of urea water 10. The reactor is provided with: a plurality of flow straightening vanes 12 which are arranged along a direction substantially bisecting an angle formed by an exhaust gas 1 inflow direction (refer to the x arrow) and outflow direction (refer to the y arrow) and which are installed in an orientation aligned with the exhaust gas 1 flow; and a support body 13 which is housed inside the gas collection chamber 7A with the flow straightening vanes 12 supported thereon. Each of the flow straightening vanes 12 is supported by the support body 13 so as to be spaced away from flow path walls of the gas collection cha
