To safely obtain the subject high-purity purified compound according to simple operations by reacting a slurry of a crude azobis-based compound containing HCN as an impurity with NH4+ ion and an aliphatic aldehyde and reducing the HCN concentration.

A crude azobis-based compound represented by the formula (R1 and R2 are each H, a halogen, a 1-4C alkoxy, etc.) is reacted with an NH4+ ion, e.g. (MH4)2SO4 and a 1-5C aliphatic aldehyde, e.g. formalin and treated to provide an amino-nitrile. The amounts of the aldehyde and NH4+ ion used are usually 3-5 equiv. based on the HCN. The treating temperature is 5-85°C and the treating time is usually within 4hr. The pH at the time of the treatment is usually ≤6, preferably 0.5-2.5. The resultant compound, represented by the formula and having a reduced content of the HCN is treated by usual operations such as extraction, washing or recrystallization to afford the purified compound represented by the formula.