A reactor comprising: a reaction-side flow passage through which a fluid as a reaction object flows; and a catalyst structure installed in the reaction-side flow passage, the catalyst structure comprising: a body part formed as a corrugated plate shaped metal plate configured with raised parts and depressed parts, the body part partitioning the reaction-side flow passage into a plurality of flow passages (A, B) disposed side by side in a direction perpendicular to a flow direction of the fluid, a catalyst carried on the body part to promote a reaction of the fluid, and communication holes to make the plurality of flow passages (A, B) partitioned by the body part communicate with each other, characterised in that the catalyst structure further includes a flat plate in contact with the depressed parts in the body part, the communication holes comprise first groove parts each provided in the raised parts and formed between each of the raised parts and a top surface of the reaction-side flow passage, and second groove parts each provided in the flat plate at a point in contact with the depressed parts of the body part; and the first groove parts and the second groove parts extend along the direction perpendicular to the flow direction of the fluid. a reaction-side flow passage through which a fluid as a reaction object flows; and a catalyst structure installed in the reaction-side flow passage, the catalyst structure comprising: a body part formed as a corrugated plate shaped metal plate configured with raised parts and depressed parts, the body part partitioning the reaction-side flow passage into a plurality of flow passages (A, B) disposed side by side in a direction perpendicular to a flow direction of the fluid, a catalyst carried on the body part to promote a reaction of the fluid, and communication holes to make the plurality of flow passages (A, B) partitioned by the body part communicate with each other, a body part formed as a corrugated plate shaped metal plate configured with raised parts and depressed parts, the body part partitioning the reaction-side flow passage into a plurality of flow passages (A, B) disposed side by side in a direction perpendicular to a flow direction of the fluid, a catalyst carried on the body part to promote a reaction of the fluid, and communication holes to make the plurality of flow passages (A, B) partitioned by the body part communicate with each other, characterised in that the catalyst structure further includes a flat plate in contact with the depressed parts in the body part, the communication holes comprise first groove parts each provided in the raised parts and formed between each of the raised parts and a top surface of the reaction-side flow passage, and second groove parts each provided in the flat plate at a point in contact with the depressed parts of the body part; and the first groove parts and the second groove parts extend along the direction perpendicular to the flow direction of the fluid. the communication holes comprise first groove parts each provided in the raised parts and formed between each of the raised parts and a top surface of the reaction-side flow passage, and second groove parts each provided in the flat plate at a point in contact with the depressed parts of the body part; and the first groove parts and the second groove parts extend along the direction perpendicular to the flow direction of the fluid.