AC impedance spectroscopy is a powerful and convenient technique to investigate the elemental reaction at electrode | electrolyte interface in rechargeable lithium ion battery. This paper describes fitting procedure of impedance plots and assignment of each separated resistance to elemental reaction. In particular, the elemental reaction occurred at A-site deficient perovskite, La_1/3NbO₃, electrode is focused in this paper, since no significant side-reaction was observed due to mild voltage range from 2 to 1 V vs. Li⁺/Li. The results indicated two elemental reactions at the interface of electrode | electrolyte corresponding to higher and lower frequency region in AC impedance spectra, and attempts has been made to be described in terms of an ‘adatom model’.