PURPOSE: To efficiently obtain a compd. useful as an intermediate for the synthesis of a cyanoguanidine as a hardening agent for a epoxy resin in a short time by subjecting a N,N'-substd. thiourea to photooxidation with oxygen in the presence of an acid scavenger.

CONSTITUTION: A thiourea expressed by the formula (R, R1 are a 1 to 20C alkyl, 3 to 10C cycloalkyl, polycyclic hydrocarbon, 6 to 14C aryl, etc.), is subjected to photooxidation with gaseous oxygen or oxygen in a mixture with inert gas in a solvent such as acetonitrile in the presence of a soluble acid scavenger (e.g. NaOH). The photooxidation is carried out by using UV rays or visible rays in the presence of a sensitizer (e.g. methylene blue) at -20 to 50°C to obtain a carbodiimide expressed by R-N=C=N-R1. At least 2 equiv. of the acid scavenger is used to 1 mole of the thiourea.