A diagnostic device diagnoses a rechargeable lithium ion battery that is configured of a plurality of battery blocks connected in series for a degraded state attributed to metallic lithium deposited therein. If a condition is established for starting a diagnosis, the diagnostic device starts discharging the battery, and after each battery block has a voltage (Vb) decreased to a measurement starting voltage (Vs) until the block has the voltage (Vb) decreased to a measurement ending voltage (Ve) the block discharges an amount (Q) of power, which the diagnostic device calculates for the battery block. If all blocks each discharge power in an amount (Q) larger than a brand-new product threshold value (A1) the diagnostic device determines that the battery is in a brand new condition. If all blocks each discharge power in an amount (Q) larger than a uselessness threshold value (A2) the diagnostic device determines that the battery is useful. If at least one block discharges power in an amount (Q) smaller than the uselessness threshold value (A2) the diagnostic device determines that the battery is useless.