An earth fault detection apparatus includes a switch group configured to switch between a first measurement path including a battery and a capacitor, a second measurement path including the battery, a negative-side insulation resistance, and the capacitor, a third measurement path including the battery, a positive-side insulation resistance, and the capacitor; and a control unit configured to calculate a reference value, based on each charging voltage of the capacitor in each measurement path, and configured to calculate an insulation resistance with reference to a predetermined conversion map, in which the conversion map includes a conversion map corresponding to a capacitance of the capacitor, and the control unit estimates the capacitance of the capacitor, and refers to the conversion map corresponding to the capacitance of the capacitor that has been estimated.