A method for producing an isocyanate, from using an organic primary amine, urea and an organic hydroxy compound as raw materials, comprising: a carbamation step of generating an N-substituted carbamate from the organic primary amine, the urea and the organic hydroxy compound according to a carbamation reaction, and then recovering a first gaseous phase component containing the urea and/or a compound having a carbonyl group derived from the urea, the organic hydroxy compound, and ammonia; a condensation step of condensing the first gaseous phase component with a condenser; an isocyanate production step of producing an isocyanate by subjecting the N-substituted carbamate generated in the carbamation step to pyrolysis; a regeneration step of allowing a part or the whole of a first residual liquid obtained in the isocyanate production step, from which low-boiling components containing the isocyanate and the organic hydroxy compound obtained in the isocyanate production step have been separated, to react with urea and an organic hydroxy compound to regenerate N-substituted carbamate; wherein the first residual liquid comprises polymeric by-products generated as a result of side reactions induced by the N-substituted carbamate and/or the isocyanate; wherein the polymeric by-products include a compound having a ureylene group or a compound having a biuret group; a separation step of subjecting the reaction solution in the regeneration step containing the regenerated N-substituted carbamate to a pyrolytic reaction, so as to separate the generated isocyanate from a second residual liquid containing a non-regeneratable by-product; and a blow-down step of heating the first residual liquid and/or the second residual liquid to recover low-boiling components containing an organic hydroxy compound, recycling the low-boiling components to at least one of the carbamation step, the isocyanate production step and the regeneration step, and removing high-boiling components containing non-regeneratable by-products from the system, wherein the organic hydroxy compound is an aromatic hydroxy compound. a carbamation step of generating an N-substituted carbamate from the organic primary amine, the urea and the organic hydroxy compound according to a carbamation reaction, and then recovering a first gaseous phase component containing the urea and/or a compound having a carbonyl group derived from the urea, the organic hydroxy compound, and ammonia; a condensation step of condensing the first gaseous phase component with a condenser; an isocyanate production step of producing an isocyanate by subjecting the N-substituted carbamate generated in the carbamation step to pyrolysis; a regeneration step of allowing a part or the whole of a first residual liquid obtained in the isocyanate production step, from which low-boiling components containing the isocyanate and the organic hydroxy compound obtained in the isocyanate production step have been separated, to react with urea and an organic hydroxy compound to regenerate N-substituted carbamate; wherein the first residual liquid comprises polymeric by-products generated as a result of side reactions induced by the N-substituted carbamate and/or the isocyanate; wherein the polymeric by-products include a compound having a ureylene group or a compound having a biuret group; a separation step of subjecting the reaction solution in the regeneration step containing the regenerated N-substituted carbamate to a pyrolytic reaction, so as to separate the generated isocyanate from a second residual liquid containing a non-regeneratable by-product; and a blow-down step of heating the first residual liquid and/or the second residual liquid to recover low-boiling components containing an organic hydroxy compound, recycling the low-boiling components to at least one of the carbamation step, the isocyanate production step and the regeneration step, and removing high-boiling components containing non-regeneratable by-products from the system, wherein the organic hydroxy compound is an aromatic hydroxy compound.