A battery management device is provided for accurately measuring in a simple manner the internal resistance (reciprocal of conductance) of a battery that serves as the determination reference of the deterioration level of a battery without the application of an AC voltage to the battery while the battery remains onboard, and determining and displaying the deterioration level of the onboard battery. The onboard battery management device has sensors 2 and 3 for measuring terminal voltage and current of an onboard battery 1; a computation processor 6 for determining the deterioration level of a battery (as well as for calculating and determining the internal resistance on the basis of the measured value); and a display unit 7 of the battery state, wherein the internal resistance is determined based on an AC voltage component and an AC electric current component that are calculated by performing differential processing or subtractive processing on the voltage value and electric current value of the onboard battery during engine operation measured without the application of an AC signal from a sensor terminal; and the DC component, the low-frequency component, and the high-frequency component are removed using a filter circuit to determine the deterioration level.