A degradation judgment circuit for a secondary battery used in a main apparatus includes a state data acquisition portion, an estimation portion, and a judgment portion. The state data acquisition portion acquires state data that shows the state of the secondary battery during a period from when the secondary battery is connected to the main apparatus to when the degradation of the secondary battery is judged. The estimation portion estimates (a) an internal resistance value at the time of judging degradation of the secondary battery or (b) a change in internal resistance value at the time of judging degradation of the secondary battery with respect to an initial internal resistance value at the time of connecting the secondary battery to the main apparatus, based on the state data. The judgment portion judges the degradation of the secondary battery based on the result of the estimation by the estimation portion.