PURPOSE: To obtain the subject compound useful as an intermediate for medicines, agricultural chemicals, etc., in high conversion rate and quality without requiring recovery of the unreacted raw materials, by dividedly adding a relatively small excessive amount of a methylenating agent to an N- phenylcarbamate in the presence of an acid catalyst.

CONSTITUTION: A methylenating agent (e.g., trioxane or paraformaldehyde) is reacted with an N-phenylcarbamate in the presence of an acid catalyst (e.g., H2SO4, HCl, phosphoric acid, polysulfuric acid or polyphosphoric acid) to provide polymethylene polyphenyl polycarbamate. In the process, the methylenating agent is used in an excessive amount of ≤twice based on the stoichiometric amount and added in ≥2 divided portions. In the divided addition, an almost stoichiometric amount (0.5mol. methylenating agent) or a somewhat smaller amount (e.g., 0.7W1.0 based on the stoichiometric amount) is previously added and the rest is added after a time of about half the total reaction time elapsed.