PURPOSE: To enable to readily separate a high boiling point solvent from a mixture of the high boiling point solvent and high boiling point products (high boiler) by-produced in the reaction, when an urethane compound is subjected to a thermal decomposition distillation in the coexistence of the high boiling point solvent.

CONSTITUTION: A solution discharged from the bottom of a reactor in which a thermal decomposition reaction was carried out is treated with an alcohol to separate the high boiling point solvent from polymers by-produced in the thermal decomposition reaction. 13-13 is a reactor for the alcohol treatment. The high boiling point solvent can be economically recovered in a good yield from a mixture of the high boiling point solvent and the polymer produced when the urethane compound is thermally decomposed in the high boiling point solvent.