NOVELTY - The diagnostic apparatus has wirings (38b,38a) for applying voltage and addition direct current (DC) power supply to solar cell module string (16) of solar cell array (12) during charge and discharge of input capacitor (26) respectively. Two sensors (24,22) measure voltage and electric current of string when sum total voltage is applied respectively. A CPU (34) diagnoses electrical property of string based on change of coupling current-voltage characteristic which coupled current-voltage characteristics acquired from voltages and electric currents measured during wiring selections.    USE - Diagnostic apparatus of solar cell array, used in power conditioner (claimed) of solar energy power generation system.    ADVANTAGE - The charge/discharge characteristic under the weak light of the capacitor utilized at the time of electric power generation is utilized for diagnosing the electrical property of the solar cell array, the capacitance of addition DC power supply is reduced. Since the electrical property of solar cell module string is diagnosed based on the change of coupling current-voltage characteristic which coupled the current-voltage characteristics acquired from voltages and electric currents measured at the time of wiring selections, the electrical property of the solar cell array is diagnosed in real time, without preventing the electric power generation at the time of sunlight irradiation. Hence, the failure of the solar cell array is discovered at early stage. The structure of the solar cell array diagnostic apparatus is simplified, thus the cost of the diagnostic apparatus is reduced.    DETAILED DESCRIPTION - The CPU of diagnostic apparatus selects primary wiring, when the output of the solar cell array is below threshold value. The CPU selects secondary wiring while measuring the voltage and electric current of the string at the time of charge of the capacitor. INDEPENDENT CLAIMS are included for the following:    (1) diagnostic method of solar cell array; and    (2) program for diagnosing solar cell array.    DESCRIPTION OF DRAWING(S) - The drawing shows a circuit block diagram of the solar energy power generation system. (Drawing includes non-English language text)    Solar cell array (12)    Solar cell module string (16)    Current sensor (22)    Voltage sensor (24)    Input capacitor (26)    CPU (34)    Wirings (38a,38b)