A charged state detecting apparatus for a secondary battery is provided, which can suppress deterioration in the accuracy of detecting a charged state, such as an SOC, the deterioration being caused by variation of a polarization state of a battery. The charged state detecting apparatus stabilizes an amount of polarization of the battery, stops power generation upon confirmation of the stabilization of the polarization amount, and sufficiently alters the current of the battery to sample a required number of data pairs of voltage and current of the battery. Using these data pairs, the charged state detecting apparatus detects a charged state, such as the SOC, of the battery.