PURPOSE: To reduce initial and running costs by incorporating a cooler tube bundle in a reactor so that a titled device is made compact and a large temp. difference between the inside and outside of a reaction tube is taken.

CONSTITUTION: A cooler tube bundle 12 for a heat medium is provided in the central part in a reactor 11 of a multitube type exothermic reaction device which removes the reaction heat in reaction tubes 13 by the circulation of the heat medium. The tube bundles 13 are disposed around said tube bundle and a baffle plate group which has the plane rectangular to the tube bundles and is disposed alternately with large diameter hollow discs 15, small diameter hollow discs 16 and middle diameter solid discs 17 provided on the same plane. As a result, the heat medium in the reactor 11 is partly cooled many times in the midway of circulation and has about the same temp. as the temp. in the stage of departing by mixing with the heat medium having an increased temp. as it passes through the clearances. If the number of the passage is increased, the higher difference between the inside and outside temp. in the reaction tube is taken. The device is thus made compact and the initial and running costs are reduced.