The possibility of a paleotemperature proxy, skeletal magunesium/strontium (Mg/Sr) ratio analyzed using ICP-AES technique, was examined on a modern Porites choral skeleton from Sesoko Island, the Ryukyus. The Mg/Sr ratio showed clear cyclic variations along a growth axis in the skeleton, fluctuating between 0.40 ∼ 0.59 (atomic ratio). The annual nature of the variations was confirmed by comparison with annual banding observed in X-radiographs, together with density measurements. Variations in Mg/Sr ratio were in phase with those in δ¹⁸O of skeletal aragonite. Given a calendar in the skeleton based on temperature dependency using annual δ¹⁸O variations, Mg/Sr ratio is strongly correlated with sea surface temperature, suggesting that the ratio have a potential as a paleo-thermometer. Mg/Sr ratio is an good proxy of temperature, compared to conventional Mg/Ca and Sr/Ca ratios due to the convination effect of directly opposed parameters. Mg/Sr ratio also has an advantage in measurements, because there is no necessity of calcium measurements, which might cause some trouble due to small amplitude of the variation when Ca is dominant. The Mg/Sr ratio using a convenient ICP-AES technique, yielding a precision of ±1.02% (2σ) for Mg/Sr ratio, enables to reconstruct paleo-temperature within the precision of ±0.36°C.