A reactor for allowing a catalyst for accelerating reaction of a reactant to act on a reaction fluid having a reactant, the reactor comprising: a partition (112, 512, 612) 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 the catalyst and being respectively provided in each of the plurality of reaction flow passages, characterised in that: the partition (112, 512, 612) has a communicating portion (350, 450, 550, 650) allowing the plurality of reaction flow passages to communicate with each other, the communicating portion (350, 450, 550, 650) is arranged in a location closer to the inlet side branch points than to the outlet side branch points of the reaction flow passages within the range where the reaction fluid branches to flow; each of the plurality of catalyst structures is loaded into each of the plurality of reaction flow passages, and has a main body made of a thin plate material bent in a corrugated shape on which the catalyst is supported, the main body of each of the plurality of catalyst structures divides the reaction flow passage along the flow direction of the reaction fluid and defines a plurality of partial flow passages in parallel, and the main body has a communicating portion allowing the plurality of partial flow passages to communicate with each other and being formed to overlap at least partially with the communicating portion (350, 450, 550, 650) of the partition. a partition (112, 512, 612) 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 the catalyst and being respectively provided in each of the plurality of reaction flow passages, characterised in that: the partition (112, 512, 612) has a communicating portion (350, 450, 550, 650) allowing the plurality of reaction flow passages to communicate with each other, the communicating portion (350, 450, 550, 650) is arranged in a location closer to the inlet side branch points than to the outlet side branch points of the reaction flow passages within the range where the reaction fluid branches to flow; each of the plurality of catalyst structures is loaded into each of the plurality of reaction flow passages, and has a main body made of a thin plate material bent in a corrugated shape on which the catalyst is supported, the main body of each of the plurality of catalyst structures divides the reaction flow passage along the flow direction of the reaction fluid and defines a plurality of partial flow passages in parallel, and the main body has a communicating portion allowing the plurality of partial flow passages to communicate with each other and being formed to overlap at least partially with the communicating portion (350, 450, 550, 650) of the partition. the partition (112, 512, 612) has a communicating portion (350, 450, 550, 650) allowing the plurality of reaction flow passages to communicate with each other, the communicating portion (350, 450, 550, 650) is arranged in a location closer to the inlet side branch points than to the outlet side branch points of the reaction flow passages within the range where the reaction fluid branches to flow; each of the plurality of catalyst structures is loaded into each of the plurality of reaction flow passages, and has a main body made of a thin plate material bent in a corrugated shape on which the catalyst is supported, the main body of each of the plurality of catalyst structures divides the reaction flow passage along the flow direction of the reaction fluid and defines a plurality of partial flow passages in parallel, and the main body has a communicating portion allowing the plurality of partial flow passages to communicate with each other and being formed to overlap at least partially with the communicating portion (350, 450, 550, 650) of the partition.