Provided is a reactor in which a catalyst to accelerate reaction of a reactant is allowed to act on a reaction fluid having the reactant. The reactor has a partition that defines, in a parallel form, a plurality of reaction flow passages through which the reaction fluid flows, and a plurality of catalyst structures, each having a catalyst and being respectively provided in each of the plurality of reaction flow passages. The partition has a communicating portion allowing the plurality of reaction flow passages to communicate mutually.