An analytical technique is presented for isotope analysis of a Sn ore mineral, cassiterite. Hydroiodic acid was used to successfully decompose the cassiterite samples. An extraction chromatographic material, TRU resin was utilized for the chemical purification of Sn. Although it has been confirmed that this purification process causes isotopic fractionation, the effect can be corrected. The purification method was used to investigate natural isotopic variations in Sn in cassiterite from Asian countries (Japan, China, Thailand, and Malaysia), via multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS). Consequently, isotopic variations as large as 0.77‰ in 124Sn/120Sn were observed for cassiterite samples from Japan and China. The isotopic fractionations of all the samples were dependent on isotope mass. However, the cause of Sn isotopic variation is yet to be established.