A battery diagnostic apparatus and a battery diagnostic method that are capable of accurately determining a deteriorated state of a battery and enabling use of the battery immediately before or near the end of the battery life are provided. The battery diagnostic apparatus includes a power supply monitoring unit 5 configured to detect that a power supply voltage has changed from less than a predetermined voltage to equal to or more than the predetermined voltage, and a controller 4 configured to measure a battery voltage in a predetermined period after the detection that the power supply voltage is equal to or more than the predetermined voltage, calculate electric power associated with a remaining capacity of the battery 1 based on the battery voltage, and perform deterioration determination of the battery 1 based on the electric power associated with the remaining capacity of the battery 1.