An electrolytically decomposing method of an iron-cyanide complex which includes the steps of introducing an electrolyte containing the iron-cyanide complex into an electrolytic cell, applying a voltage between the electrodes to electrolytically decompose the iron-cyanade complex and oxidizing cyan to form a cyanic acid while precipitating an iron hydroxide is disclosed. In the method a specific conductivity of the electrolyte is adjusted to not more than 30 m /cm.