In a ground fault detection device, two photovoltaic strings are disconnected from a photovoltaic power generation system, a measurement of a first voltage value is performed for the one photovoltaic string by a first measurement unit, and in parallel therewith, a measurement of a second voltage value is performed for the other photovoltaic string by the second measurement unit by executing various functions of a calculation control unit. Thus, a capacitance relative to a ground of a measurement target is reduced. Further, the photovoltaic string is electrically separated from a load device at the time of such a measurement. Further, separate measurements are performed on the one and other photovoltaic strings in the disconnected state, and the first and second voltage values are measured in parallel.