Sudden and large changes in renewable energy generators called ramp events are one of the major issues to be addressed for the stable operation of electric power system. By classifying the change of spatial average irradiance in a day, which is the primary factor of aggregated power output of photovoltaic power generation system (PV system), this study statistically evaluated the frequency of ramp events in a year. As a result for the spatial average irradiance in the Chubu area in Japan, the ramp event occurs in 42 days (12%) in a year. The duration of ramp event varies mainly between 30-120min. Then, this study statistically evaluated the short-cycle fluctuation characteristics of spatial average irradiance during ramp event. As a result, in the case of ramp down of relatively large width, the short-cycle fluctuation before and during the ramp event is as large as that in quasi-fine day, while it after the ramp event is small enough as in cloudy day. The results can be useful as approximate evaluation regarding generation flexibility required in electric power system with high penetration PV systems.