In this paper, we propose a new method to detect defective string(s) in a photovoltaic (PV) array, in which the temporal variation of string current is employed since it is easily measured in generation mode, independently of the scale of photovoltaic systems. Based on the results of experimental works using both a newly-installed and 20-years-old PV systems, it is demonstrated that the amplitude of string current variation corresponds to the voltage-current characteristics of the target string at the operation voltage of power conditioning system. Numerical results using a simple PV array model quantitatively confirm the experimental results. Two parameters are newly proposed to visualize the measured data. It is illustrated that those two parameters are useful to this end, and consequently, make it possible to easily evaluate the conditions of target string such as power loss and cluster loss.