A secondary battery deterioration degree calculating apparatus comprising: a stop-time voltage obtaining section for obtaining a voltage value at a point of time when charge and discharge of a target secondary battery is terminated; a start-time voltage obtaining section for obtaining a voltage value at a point of time when charge and discharge of the target secondary battery is started for the first time after the stop-time voltage obtaining section obtains data; an SOC obtaining section for obtaining an SOC of the target secondary battery at a stop time or at a start time; a time obtaining section for obtaining a length of time (hereinafter, referred to as "elapsed time") from when the stop-time voltage obtaining section obtains the data to when the start-time voltage obtaining section obtains data (hereinafter, referred to as "unused period"); a self-discharge slope obtaining section for obtaining a self-discharge slope of the target secondary battery by dividing an absolute value of a difference between the start-time voltage value obtained by the start-time voltage obtaining section and the stop-time voltage value obtained by the stop-time voltage obtaining section by the elapsed time obtained by the time obtaining section; a self-discharge map in which a relationship between an SOC and the self-discharge slope in the target secondary battery is recorded for each temperature; a temperature determining section for obtaining a temperature from the self-discharge slope obtained by the self-discharge slope obtaining section, the SOC obtained by the SOC obtaining section, and the self-discharge map; a deterioration degree calculating section for calculating a progress degree of deterioration of the target secondary battery during the unused period based on the temperature obtained by the temperature determining section and the elapsed time obtained by the time obtaining section; and a deterioration degree accumulating section for accumulating the progress degree of deterioration calculated by the deterioration degree calculating section. a stop-time voltage obtaining section for obtaining a voltage value at a point of time when charge and discharge of a target secondary battery is terminated; a start-time voltage obtaining section for obtaining a voltage value at a point of time when charge and discharge of the target secondary battery is started for the first time after the stop-time voltage obtaining section obtains data; an SOC obtaining section for obtaining an SOC of the target secondary battery at a stop time or at a start time; a time obtaining section for obtaining a length of time (hereinafter, referred to as "elapsed time") from when the stop-time voltage obtaining section obtains the data to when the start-time voltage obtaining section obtains data (hereinafter, referred to as "unused period"); a self-discharge slope obtaining section for obtaining a self-discharge slope of the target secondary battery by dividing an absolute value of a difference between the start-time voltage value obtained by the start-time voltage obtaining section and the stop-time voltage value obtained by the stop-time voltage obtaining section by the elapsed time obtained by the time obtaining section; a self-discharge map in which a relationship between an SOC and the self-discharge slope in the target secondary battery is recorded for each temperature; a temperature determining section for obtaining a temperature from the self-discharge slope obtained by the self-discharge slope obtaining section, the SOC obtained by the SOC obtaining section, and the self-discharge map; a deterioration degree calculating section for calculating a progress degree of deterioration of the target secondary battery during the unused period based on the temperature obtained by the temperature determining section and the elapsed time obtained by the time obtaining section; and a deterioration degree accumulating section for accumulating the progress degree of deterioration calculated by the deterioration degree calculating section.