The diagnostic method for a photovoltaic power system includes: a step of storing measured values of voltage and current outputted by a photovoltaic cell array, and identifying a first time period and a second time period with a lower solar radiation amount than in the first time period; a step of calculating the number of faulty photovoltaic cells on the basis of the measured values in the first time period; a step of calculating the number of faulty photovoltaic cells on the basis of the measured values in the second time period; and a step of comparing the number of faulty photovoltaic cells as calculated for the first time period, and the number of faulty photovoltaic cells as calculated for the second time period, and identifying a series resistance of the photovoltaic cells and a value indicating a degree of loss resulting from a fault in the photovoltaic cells.