The purpose of this paper is to: (1) collect relevant data and estimate Japanese international financial assistance for HIV/AIDS control; (2) discuss the difficulties in collecting relevant data and the limitations of the collected data; and (3) conduct a comparative analysis on the estimated data with OECD and Kaiser Family Foundation aggregate data. The point is that we have comprehensively collected and estimated the data on Japanese international expenditures for HIV/AIDS control while there is no reliable data that is totally managed and published. In addition, we discuss the difficulties and limitations of data collection: unpublished data; insufficient data; inseparable data; problems of exchange rates; gaps between disbursement and commitment; and difference in year period among calendar, fiscal and organization-specific years. Furthermore, we show the risk of underestimating the Japanese international contribution to HIV/AIDS control on the basis of OECD and Kaiser data. In this respect, it is significant to comprehensively collect and estimate the data on Japanese international assistance for HIV/AIDS control. Finally, we derive the implication that it is crucial for a relevant international organization and/or individual countries to comprehensively collect and administer data for international cooperation in the development of health policies for HIV/AIDS.