This study aims to obtain the rate of rainfall (RR) over ocean areas using satellite data by establishing the relationship between Advanced Microwave Sounding Unit-A (AMSU-A) microwave brightness temperature (Tbs) data and island rain gauge rainfall. This study applied a rain identification technique developed by Grody et al. (1999) using AMSU-A data accepted to screen rain events over ocean areas. Using rain gauge data of Sakishima-shoto in Japan as a ground truth, validation of rain identification was conducted every May and June from 1999 to 2001. Including times of both rainfall and no rainfall, the overall rates of successful recognition were 87.3%, 87%, and 89% for 1999, 2000, and 2001, respectively. As for quantitative validation, the coefficient of determination was 0.72. To obtain further precipitation observations, another rainfall estimation retrieved by the TRMM Microwave Imager (TMI) data onboard the Tropical Rainfall Measuring Mission (TRMM) was compared with the AMSU-A data. The results displayed solid agreement in rainfall patterns and rain distribution. Additionally, when the AMSU-A RRs were multiplied by a factor of approximately 1.5 (particularly for heavy rainfalls), the intensities of the AMSU-A RRs roughly equaled those of the TMI RRs. Combining this data with the other satellite rainfall retrieval data, a series of AMSUA rain maps for sea areas were shown to mitigate the problem of relatively sporadic observations of rainfall at sea.