One of the serious fearsfor Japanese societyiscontamination of radioactive substancesdue to the hugeearthquake and subsequentFukushima No. 1 nuclear powerplant disaster. This paper proposes a detection method to identifydiffusion sources of radioactive small particles in the air based onpublicly available data, which are composed ofair dose rate,amount ofrain, wind speed,and direction.Air dose rate isobserved on each public monitoring point. The nearest weatherobservation station for each public monitoring point concerningair dose rate is also identified to analyze the relationship betweenair dose rate and weather conditions. This method focuses on allcases of continuous rainfall duration, because various sizes ofspike concerning air dose rate on a public monitoring point areobserved among the cases. Each spike starts when rainfall beginsand the spike disappears when rainfall continues. This is becauserainfall cleans up radioactive particles in the atmosphere. Themethod confirmsastatistically significant difference of increaserate of air dose rate between each pair among rainfall cases. Italso identifies an existence range of direction of diffusion sourcesbased on significance tests of correlation coefficients.