According to an aspect of the present invention, a battery diagnosis device for a battery including a parallel connection body of a plurality of unit cells comprises: a data acquisition unit configured to collect charging and discharging data including a voltage time series indicating a change in a voltage across both end terminals of the battery over time and a current time series indicating a change in a charging and discharging current flowing through the battery over time; and a data processing unit configured to determine an estimated capacity value indicating a full charge capacity of the battery on the basis of the charging and discharging data. The data processing unit is configured to monitor a change in the estimated capacity value over time to diagnose any abnormality of the parallel connection body.