A ground-fault determination apparatus, which is for use in a vehicle power supply system including a DC power source and a DC supply circuit, includes an acquisition unit that applies an AC signal to the DC supply circuit through a series connection of a resistor and a capacitor to acquire a peak value of the AC signal divided by a resistance of the resistor and an insulation resistance of the DC supply circuit, a ground-fault determination unit that determines presence of a ground-fault based on a comparison between a determination threshold and the peak value, an estimation unit that estimates at least one of a maximum output voltage of the DC supply circuit and a common capacitance between the DC supply circuit and a vehicle body, and a setting unit that sets the determination threshold based on at least one of the maximum output voltage and the common capacitance.