Recently，large-scale photovoltaic (PV) systems have attracted attention. In the field-operation，the electric power generations of PV systems are affected by the natural environment such as ambient temperature，snowfall，soiling，etc. The influence of the unusual natural environment such as volcanic activity has not been reported. At Miyazaki，southern Japan，Shinmoedake volcano has been in a volcanic-activity since 2010. In this study，we assessed the impact of the volcanic ash on the CIGS PV system that had two tilt angles. The output of CIGS PV system was drastically decreased by the accumulation of volcanic ash. Before the ash fall，the average performance ratio (PR) of the CJGS system was approximately 0.90. The PR was drastically decreased to 0.30 by the volcanic ash. However，the PR recovered after the rainfall. The influence of tilt angle on the recovery of PR was also asessed. It was found that the larger tilt angle had an advantage for the recovery of PR because the volcanic ash could be easily washed out.