A secondary battery restoring method includes a first step of determining whether or not a value of an input-output characteristic of a secondary battery or a battery module is within a predetermined range of reference values, and a second step of adjusting the secondary battery or the battery module to a predetermined low SOC condition of from 0% to 20% SOC and thereafter leaving the secondary battery or the battery module to stand if, in the first step, the value of the input-output characteristic is determined to be outside the predetermined range of reference values.