PURPOSE: To offer a cooling device in which there is no fear that water is split around a reaction container by a method wherein the reaction container is cooled via the water which has been stored between the outer circumference face of the reaction container and a cover surrounding the container.

CONSTITUTION: A cover 2 which nearly coaxially surrounds a cylindrical reaction container 1 composed of quartz glass is made of a stainless steel sheet; a flexible steel pipe 15 is wound along its outer circumference face as a cooling pipe in which cooling water is passed. Water is stored between the cover 2 and the reaction container 1; the cooling water which flows into from a cooling- water entrance flows in the cooling pipe 15 at the outer circumference of the cover, cools the cover 2 and, after that, is discharged to a cooling-water exit through a flow meter 4 and a flow-rate regulating valve 5. The water which has been stored between the reaction container 1 and the cover 2 is moved upward along the outer circumference face of the heated reaction container; while it is being moved downward along the inner wall face of the cooled cover, it is circulated by convection and cools the reaction container 1 form the side of the outer circumference face.