In a status detector for a power supply, a power supply, and an initial characteristic extracting device for use with the power supply, a measuring unit obtains measured values of at least current, voltage and temperature of the electricity accumulating unit. A processing unit executes status detection of the electricity accumulating unit by using the measured values and the characteristic information of the electricity accumulating unit which is stored in a memory unit. A discrepancy detecting unit detects the presence of a discrepancy away from a theoretical value when a result of the status detection is changed over a predetermined threshold or reversed with respect to the measured values. A modifying unit modifies the characteristic information depending on the detected discrepancy.