Treering δ13C of both modern Japanese cedars, and of a giant Japanese cedar growing throughout the last two millennia on Yakushima Island, southern Japan, were studied. δ13C of eleven modern trees from 320 m to 1700 m above mean sea level revealed a strong dependency on tree growth‐elevation and a decreasing trend from the late 19th century to the present. Changes in both climate and δ13C of atmospheric CO2 appear to be important for these changes. Assuming temperature change to be a dominant factor influencing δ13C‐growth elevation relationship, we have inferred a temperature dependency of δ13C, −0.29±0.02‰ °C−1 for Japanese cedar. Applying this relationship to a decadal‐δ13C time series of a giant Japanese cedar, we reconstructed a high‐resolution climatic record for the last two millennia. A maximum‐entropy spectral analysis of the tree‐ring δ13C time series revealed a resemblance of that of other climatic proxy, and the presence of the well‐known periodicity (ca 90 and near 200 years), characteristic of tree‐ring Δ14C, governed by solar modulation of galactic cosmic rays. Supporting evidence for a Sun‐climate relationship was presented.