An earth fault detection apparatus to be connected with a non-grounded battery and for calculating an insulation resistance of a system including the battery, the earth fault detection apparatus comprising: a capacitor configured to be operated as a flying capacitor; a switch group (S1-S4) configured to switch between a first measurement path, a second measurement path, and a third measurement path, the first measurement path including the battery and the capacitor, the second measurement path including the battery, a negative-side insulation (RLn) resistance that is an insulation resistance between a negative-side line of the battery and ground, and the capacitor, the third measurement path including the battery, a positive-side insulation resistance (RLp) that is an insulation resistance between a positive-side line of the battery and the ground, and the capacitor; a reference resistance (Rref) and a test switch (Sr) connected in series between the positive-side line or the negative-side line of the battery and the ground; and a control unit configured to calculate a first reference value based on each charging voltage in a case where the test switch (Sr) is opened and the capacitor is charged for a first time in each of the first to third measurement paths, and configured to calculate the insulation resistance with reference to a conversion map that has been created to correspond to an electrostatic capacitance (CYp, CYn) between the ground and one of the positive-side line and the negative-side line of the battery, wherein the control unit calculates a second reference value based on each charging voltage in a case where the test switch (Sr) is closed and the capacitor is charged for a second time in each of the first to third measurement paths, the second time being shorter than the first time, and configured to estimate the electrostatic capacitance (CYp, CYn) with reference to a predetermined test conversion map. a capacitor configured to be operated as a flying capacitor; a switch group (S1-S4) configured to switch between a first measurement path, a second measurement path, and a third measurement path, the first measurement path including the battery and the capacitor, the second measurement path including the battery, a negative-side insulation (RLn) resistance that is an insulation resistance between a negative-side line of the battery and ground, and the capacitor, the third measurement path including the battery, a positive-side insulation resistance (RLp) that is an insulation resistance between a positive-side line of the battery and the ground, and the capacitor; a reference resistance (Rref) and a test switch (Sr) connected in series between the positive-side line or the negative-side line of the battery and the ground; and a control unit configured to calculate a first reference value based on each charging voltage in a case where the test switch (Sr) is opened and the capacitor is charged for a first time in each of the first to third measurement paths, and configured to calculate the insulation resistance with reference to a conversion map that has been created to correspond to an electrostatic capacitance (CYp, CYn) between the ground and one of the positive-side line and the negative-side line of the battery, wherein the control unit calculates a second reference value based on each charging voltage in a case where the test switch (Sr) is closed and the capacitor is charged for a second time in each of the first to third measurement paths, the second time being shorter than the first time, and configured to estimate the electrostatic capacitance (CYp, CYn) with reference to a predetermined test conversion map.