PURPOSE: To make it possible to obtain resin varnish with a constant quality, by forming the inner wall of a reaction vessel for the reactive preparation of thermosetting resin varnish into a specific smooth finished surface.

CONSTITUTION: The inner wall of a reaction vessel for the reactive preparation of thermosetting resin varnish is formed into a smooth finished surface with sand paper of #160 or more. As the result, the adhesion of scale is made difficult to prevent the irregularity in heat conductivity in the wall surface thereof and the dehydration time of the resin varnish is prevented from becoming irregular to make it possible to obtain the resin varnish with constant quality.