A reactor including a reactor vessel and heat exchange tubes provided in the reactor vessel. The reactor vessel includes a tubesheet and is configured to receive a reaction fluid. The tubesheet has a first plate member configured to contact the reaction fluid and a second plate member configured to not contact the reaction fluid. Heat exchange tubes are provided in the reactor vessel and fixed to the first plate member. The heat exchange tubes are configured to receive a heat exchange medium. At least a portion of the first plate member configured to contact the reaction fluid is made of a metal that has a high corrosion-resistance against the reaction liquid, and the second plate member is made of a metal that has a low corrosion-resistance against the reaction liquid. The second plate member is detachably fixed to a remainder of the reactor vessel.