The analysis of the degradation mechanism of all-solid-state lithium polymer secondary batteries was carried out by using a combination of several electrochemical measurement methods. Although there have been few reports about the technique of identifying various resistances (electrolyte bulk, anode–electrolyte interface, cathode–electrolyte interface), it has become possible to quantitatively separate the resistances in the all-solid-state lithium polymer secondary battery. As a consequence, it was clarified that in the battery, the degradation of the LiCoO2 cathode–solid polymer electrolyte interface is dominant.