A new quantitiative analytical method of prussic acid by "Microdiffusion analysis" was studied. HCN-Glucoside of ARMENIACA was hydrolysed with KOH in out-room of unit, and then concentrated sulfuric acid was poured in order to liberate the HCN gas. The liberated gas was absorbed into nickel sulfate solution of inner room of unit quantitatively. The excess of nickel sulfate was determined by EDTA Reagent using MX-indicator. By this method, the following results were obtained: (1) It was needed more than 4 hours, in order to hydrolyse completely at $50^{circ}C$, but could be shortened to 3 hrs. at $^60{circ}C$. (2) It was completely absorbed into nickel sulfate solution after 30min.