Localized torrential rainfall caused heavy rainfall disasters in Summer and Baiu season of Japan called as "Guerrilla-heavy-rainfall" in Japanese media.The rainfall was produced by an isolated cumulonimbus that grew rapidly.To prevent these disasters, Japanese government recently installed networks of X-band polarimeric Doppler radars over major urban areas in Japan.X-band radar is suitable to realize earlier and more detailed detection of the baby cell, because X-band radar has higher sensitivity and spatial resolution.This research investigates the growth of the baby rain cell to become large cumulative clouds by focusing on its vorticity using Doppler velocity.In this research, we define two vortexes with different size, and evaluate a relationship qualitatively between rainfall and the existence of vortex.