Abstract: In a process for preparing a Diels-Alder addition product of a conjugated diolefin and acrylonitrile by reacting the conjugated diolefin with acrylonitrile, a vessel type reactor is used. The pressure of the gaseous phase in the reactor is increased by using an inert gas in the reaction. A premixed solution of the conjugated diolefin or a compound which produces the conjugated diolefin in the reactor and acrylonitrile is released through an upper portion of the reactor into the gaseous phase and then dropped into a liquid phase to react. A reaction product is taken out through a lower portion of the reactor. The process can be carried out continuously in a safe manner over an extended period of time without any adhesion of an insoluble polymer to the reactor or to the inlet or outlet pipes, and without sudden pressure fluctuations.