PURPOSE: To selectively obtain isocyanate at a high yield by treating a liquid mixture composed of unchanged urethane and isocyanate while expelling alcohol of a by-product without using catalysts in this mixture, thereby decomposing the urethane.

CONSTITUTION: The unchanged urethane and isocyanate are incorporated into a high boiling inert solvent (e.g.: diphenyl ether) and the solvent is treated for 1 to 20 minutes at 220 to 350°C with a decomposition reactor. The alcohol to be by-produced is evaporated in correspondence to the decomposition rate of the urethane. The liquid mixture is then treated at 250 to 200°C with a flash device, by which the isocyanate is partially evaporated. The remaining flash liquid mixture is recirculated to the previous reactor. Distillation is then executed at 80 to 150°C, by which the isocyanate is obtd.