The interface oscillation of an encapsulated liquid drop, which comprises the outer liquid shell and the inner gas bubble, are studied experimentally and theoretically. Using a sequential production device of encapsulated drops in liquid-liquid-gas systems, the oscillating motions of encapsulated drop interfaces are observed in detail for various flow conditions. To investigate the dynamics of the interface, the oscillations of interfaces are theoretically analyzed using a model assuming small-amplitude oscillation of the interfaces based on the experimental observations. The oscillating mode, frequency and amplitude ratio between the inner and outer interfaces of the encapsulated drop are investigated quantitatively. Furthermore, the oscillation frequency of the liquid-liquid interface of the encapsulated drop in the immiscible liquid are experimentally obtained for various liquids. Comparing the theoretical results with the experimental ones, the validity of the theoretical analysis is confirmed.