Analysis is made of the 10min rainfall and air pressure by using automatic data from 76 stations, indicating that the heavy rain above the meiyu front lasts about half an hour and is concentrated in a narrow area. Rainfall of 5mm/10 min is mainly concentrated in the area of one station and occasionally spreads over three stations. Rainfall of>10 mm/10 min is observed in the area of one station and seldom covers two stations. Rainfall of>15mm/10min or 20 mm/10 min has very little probability for its occurrence. It is found that every heavy rain forms when several rain masses occur one after another and every rain mass is composed of 2-3 or more powerful rain nuclei. From the 10min variation of the greatest amplitude of air pressure at each station, the intensity and time of occurrence are well related with rainfall.