PURPOSE: To obtain the title substance which is used as a color developer for heat-sensitive paper, from readily available feedstock under mild conditions, by heating an asymmetric N,N'-diphenylthiourea in a solvent for hours in a solvent or melting it under heat to cause disproportionation.

CONSTITUTION: A compound of formula I (R1, R2 is different to each other and represent substituted or unsubstituted phenyls) is heated in a solvent such as benzene, acetone or ethanol at 50W120°C for 2W8hr to fused under heat to effect disproportionation to give the compound of formula II. The compound of formula I is readily synthesized from an isothiocyanate derivative and an aniline derivative and the product is subjected to the disproportionation without isolation advantageously through a shortened process.