A deterioration-state calculation method for a battery comprises: detecting current and voltage of the battery; calculating a charge state based on the detected value; calculating a first deterioration state of the battery for each predetermined period based on an elapsed time from the time use is started and an amount of charge-discharge; calculating a second deterioration state based on a change amount in the charge state of the battery and a change amount of current; calculating the second deterioration state for a plurality of times within the predetermined period; calculating reliability of calculation of the second deterioration state for each of a plurality of the second deterioration states based on the detected value; calculating a correction value of the deterioration state based on the second deterioration state with the reliability of the predetermined reliability or higher; and calculating the deterioration state by correcting the first deterioration state.