An electric power storage system 10 of the present invention includes a lithium secondary battery; a detection device to detect the temperature of the lithium secondary battery; and a control device to decrease a remaining capacity of the lithium secondary battery based on an increase of the detected temperature and a temperature Tbx of the lithium secondary battery. When a self-heating rate of the lithium secondary battery at a temperature T (K) and a remaining capacity x (%) is represented by Hs(x, T) (K/min), and a heat dissipation rate at a temperature T (K) of the lithium secondary battery is represented by Hd(T) (K/min), the temperature Tbx is a temperature at which Hs(x,Tbx)>Hd(Tbx) holds. That is, this temperature Tbx is a temperature at which the heat generating rate is higher than the heat dissipation rate and at which the battery temperature T starts to increase.