Rain rates are estimated from brightness temperature measured with a Microwave Scanning Radiometer (MSR) carried on board the Marine Observation Satellite 1 (MOS-1). Estimations are made using a rain rate retrieval algorithm based on a radiative-transfer model assuming rain spaced uniformly over the ocean. These values are compared with a Satellite-Derived Index of Precipitation Intensity (SI), which estimates the rain rate from visible and infrared images of a Geostationary Meteorological Satellite in conjunction with rain observation by a radar network of the Japan Meteorological Agency. Good correlation between MSR and SI derived rain rates validates the rain-rate retrieval algorithm.