PURPOSE: To prepare an organic isocyanate in high yield by thermally decomposing the corresponding carbamate in multiple steps and removing the alcohol produced by the thermal decomposition in the intermediate step of the steps by the stripping.

CONSTITUTION: The process for preparation of organic isocyanate of formula (Ar is single-substituted or multiple-substituted phenyl; R is methylene or ethylene; (n) is 0 to 2) by the thermal decomposition of the corresponding carbamate in the presence of an inert solvent comprises decomposing a carbamate 4 dissolved in a solvent into isocyanate and alcohol by the heat treatment in multiple separate reaction spaces 22, 24, 26 and 28,. stripping the alcohol by an inert stripping agent passed through stripping trays 10, 12, 14 and 16 placed at intermediate of each reaction space and by the solvent, removing the alcohol via the top of the exit tube and recovering a product solution rich in isocyanates from the lower exit tube 6.