Reactor

A reactor comprising:

• a heat exchanging body that includes therein a heat medium flow channel in which a heat medium is caused to flow, and a reaction flow channel in which a reaction fluid is caused to flow, to exchange heat between the heat medium and the reaction fluid and cause a thermal reaction in the reaction fluid to progress;
• and
• a heat transfer promoter that is provided in the heat medium flow channel and comes in close contact with the heat exchanging body in order to promote heat transfer between the heat medium and the heat exchanging body, the heat transfer promoter being constituted by an assembly of partial heat transfer promoters of a plurality of types which have heat transfer performances different from one another by type and are joined to each other in a flow direction.