In this study, we analyze the mechanism of the future change of a occurrence frequency of Guerrilla-heavy rainfall in the Kinki region in August by focusing on the future change of lapse rate and water vapor inflow using a 5km-mesh regional climate model (RCM05). From the analysis, we show the frequency of days will increase when Showalter Stability Index (SSI), which expresses atmospheric stability, becomes lower in the Kinki region in late August although lapse rate will decrease. Lower SSI means that atmosphere is unstable. Then we show that the reason of destabilization of SSI is the increase of water vapor in lower layer. Finally, we use Self-Organizing Map (SOM), which is one cluster classification method, to reveal the main reason of the increase of water vapor in the lower layer in late August. The results show the wind field which blows from the Pacific to the Kinki region is increasing, and this explains rich water vapor flux is supplied to the Kinki region.