A fault detection apparatus comprising: a sunshine condition estimation unit configured to receive an actual electrical output from a string that is a unit in which a plurality of electricity generating modules are connected in series, and output characteristic models for each electricity generating module based on which an electrical output is estimated depending on a sunshine condition affecting electricity generation, and to calculate a level of sunshine obtained by applying the actual electrical output data to the output characteristic model for each electricity generating module as a sunshine condition estimate for each electricity generating module; a sunshine condition spatial correction unit configured to calculate a first total estimate that is a sum of sunshine condition estimates of all electricity generating modules included in a target string, to calculate two second total estimates, each being a sum of sunshine condition estimates of all electricity generating modules included in each of two first adjacent strings adjacent to both lengthwise surfaces of the target string, and to correct the sunshine condition estimates of the electricity generating modules of the target string so that the first total estimate falls within a range determined by the second total estimates, wherein the adjacent strings are strings that have modules whose distances to the modules of the target string are smallest and the lengthwise surface represents a longitudinal edge for a rectangular string; and an electrical output fault detection unit configured to detect a faulty string when a difference between the actual electrical output and a sum of expected electrical outputs of the electricity generating modules calculated by using the output characteristic models and the corrected sunshine condition estimates within the target string is greater than a first threshold, and the actual electrical output is smaller than the sum of the expected electrical outputs, wherein the sunshine condition is one of a sunshine level relative to the electricity generating module, and a combination of the sunshine level and a temperature. a sunshine condition estimation unit configured to receive an actual electrical output from a string that is a unit in which a plurality of electricity generating modules are connected in series, and output characteristic models for each electricity generating module based on which an electrical output is estimated depending on a sunshine condition affecting electricity generation, and to calculate a level of sunshine obtained by applying the actual electrical output data to the output characteristic model for each electricity generating module as a sunshine condition estimate for each electricity generating module; a sunshine condition spatial correction unit configured to calculate a first total estimate that is a sum of sunshine condition estimates of all electricity generating modules included in a target string, to calculate two second total estimates, each being a sum of sunshine condition estimates of all electricity generating modules included in each of two first adjacent strings adjacent to both lengthwise surfaces of the target string, and to correct the sunshine condition estimates of the electricity generating modules of the target string so that the first total estimate falls within a range determined by the second total estimates, wherein the adjacent strings are strings that have modules whose distances to the modules of the target string are smallest and the lengthwise surface represents a longitudinal edge for a rectangular string; and an electrical output fault detection unit configured to detect a faulty string when a difference between the actual electrical output and a sum of expected electrical outputs of the electricity generating modules calculated by using the output characteristic models and the corrected sunshine condition estimates within the target string is greater than a first threshold, and the actual electrical output is smaller than the sum of the expected electrical outputs, wherein the sunshine condition is one of a sunshine level relative to the electricity generating module, and a combination of the sunshine level and a temperature.