Tree‐ring 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.