This in-vehicle DC-DC converter is configured from: a power conversion unit that transmits/receives power between a low-voltage system secondary battery and a high-voltage system secondary battery; low-voltage system AD converters and high-voltage system AD converters, which convert analog values of the currents, voltages, and temperatures of the low-voltage system secondary battery and the high-voltage system secondary battery into digital values; an input switching unit that switches analog values of the high-voltage system secondary battery into analog values of the corresponding low-voltage system secondary battery; and a calculation unit that compares the digital values of the low-voltage system AD converters and the digital values of the high-voltage system AD converters with each other. In the switched state, failure diagnosis of the AD converter is performed by comparing the digital values of the low-voltage system AD converters and the digital values of the high-voltage system AD converters with each other.