Reactor

A reactor, comprising:

• a reaction-side flow passage through which a fluid as a reaction object flows;
• and
• a catalyst structure separated from the reaction-side flow passage provided in the reaction-side flow passage,
• the catalyst structure including:
• a body part formed in a plate shape having two or more raised points and two or more depressed points each alternately provided one by one, the body part partitioning the reaction-side flow passage into a plurality of flow passages 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 partitioned by the body part communicate with each other,
• the communication holes comprising at least either:
• first groove parts each provided on a surface of each of the raised points and between each of the raised points and a wall defining the reaction-side flow passage;
• or second groove parts each provided on a surface of each of the depressed points and between each of the depressed points and the wall defining the reaction-side flow passage, each extending along the direction perpendicular to the flow direction of the fluid.