In recent years, due to the effects of global warming countermeasures and the Great East Japan Earthquake, interest in renewable energy has been increasing. In particular, there is a strong interest in Photovoltaic Power Generation (PV). Assuming before the earthquake, the goal is to increase the amount of installed PV. It is expected that the introduction of PV will be further expanded and its importance will increase at the same time. On the other hand, when PV whose output varies greatly depending on solar radiation conditions is introduced into the power system in a large amount, important issues has been pointed out that it becomes difficult to adjust the voltage of the distribution system and to secure adjustment power to absorb PV output fluctuations, and it is effect on system frequency fluctuations. In contrast, our laboratory has proposed the concept of robust reliability, which can apply the conventional N-1 reliability to uncertain environments. Furthermore, we formulated and developed a robust dynamic feasible region (RDF region), and proposed and developed a calculation method for the index that indicates the size of the RDF region. However, if this index calculation is executed in a large-scale system, there is a problem that it takes a huge amount of calculation time. Moreover, depending on the future development of robust reliability index, it is expected that the calculation will become complicated. The effect of uncertainty was evaluated, but it was not considered because there is no means to evaluate the uncertainty itself. In this paper, we formulated for the purpose of assessing uncertainty using the IGDT method.