PURPOSE: To obtain the subject compound useful as a developer of a thermal recording material in high yield and purity by reacting thiodianiline with phenylisothiocyanate in a specific molar ratio in a mixed solvent containing an aprotic polar solvent.

CONSTITUTION: Thiodianiline is made to react with phenylisothiocyanate by using 2-8 moles of the latter, preferably 2-6 moles based on 1 mole of the former to obtain 4,4'-thiobis(diphenylthiourea) of the formula in a mixed solvent containing an aprotic polar solvent. As the mixed' solvent containing the aprotic polar solvent, an N,N-dimethylformamide aqueous solution or a dimethylsulfoxide aqueous solution is preferable. By using this process, the decomposition of 4,4'-thiobis(diphenylthiourea) is suppressed, and accordingly, the objective compound is obtained in high yield and selectivity. Further, since the reaction solvent is easily recycled, the process is economical and advantageous on an industrial scale.