A deterioration degree calculating apparatus for a secondary battery of the invention includes: obtaining a voltage value at a stop time of charge and discharge of a target secondary battery; obtaining a voltage value at a first start time of charge and discharge after that; obtaining an SOC of the target secondary battery at the stop time or at the start time; obtaining a length of an unused period from the stop time to the start time (an elapsed time); obtaining a self-discharge slope of the target secondary battery by dividing an absolute value of a difference between the voltage value at the start time and the voltage value at the stop time by the elapsed time; obtaining a temperature during an unused period by use of a self-discharge map in which the SOC and the self-discharge slope is recorded for each temperature; calculating a progress degree of deterioration of the target secondary battery during the unused period based on the obtained temperature and elapsed time; and accumulating the calculated deterioration progress degree to accurately calculate the deterioration degree of the secondary battery without continuously consuming electric power during the unused period.