A SOH indicating a deterioration degree of a secondary battery is estimated. SOH information of the secondary battery is acquired. Information indicating a battery state of the secondary battery and information indicating the battery state having a correlation with the SOH higher than a predetermined correlation are acquired. A SOH estimation model is built by synthesizing a regression model using a variance-covariance matrix, in which the SOH information is defined as an output, and the information indicating the battery state that has the correlation with the SOH higher than the predetermined correlation is defined as an input The SOH is estimated by inputting information indicating a current battery state of the secondary battery into the SOH estimation model.