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Rain measurement system using propagation characteristics of microwave and millimeter wave is very effective for disaster prevention system for local strong rainfalls. Measurement technique of electromagnetic scattering and attenuation characteristics by rain is one of useful evaluation methods of rainfall rate[1,2]. Attenuations of incident wave with 20GHz carrier frequency in random media corresponding to rainfall rate 5-50 mm/h are evaluated by three-dimensional FDTD method. In FDTD analysis, incident wave with relatively flat distribution around beam center is transmitted. Firstly, we show measurement system of rain attenuation using microwave Gaussian beam. When the frequency of incident wave is 20GHz, scattering by a raindrop with 1mm diameter can be shown by Rayleigh approximation scattering theory. Next, scattered fields by single and two raindrops are evaluated by FDTD method and Rayleigh scattering theory. Lastly, scattered fields by many raindrops and specific rain attenuation are evaluated using FDTD method[4-6]. Based on these computer simulations, the optimum measurement system of rain using microwave techniques may be accomplished.