PURPOSE: To obtain guanylthiourea which is useful as an intermediate for triazine, an optical lightening agent, a plant protective, and as an ingredient of fertilizer, with excellent yield and in high quality.

CONSTITUTION: When guanylthiourea is produced by allowing dicyandiamide to react with hydrogen sulfide in the presence of a catalytically functional additive and in an N-alkylpyrrolidone as a solvent, an addition product of the objective compound and the above-mentioned solvent is formed by using an amine, ammonia and/or hydrogen sulfide as catalystically functional additives, preferably at addition amounts of 0.2-1.5 wt.% ammonia and 0.5-3.0 wt.% sulfur for the weight of dicyandiamide, and subsequently the addition compounds are treated and decomposed with an organic solvent, preferably a triol, thereby obtaining the objective compound as the residue. It is preferable that the mother liquor is recycled and that a mixture of N-methylpyrrolidone and the triol containing ammonia and sulfur element is used as a reaction solvent.