<p>Oxygen isotope ratios (δ<sup>18</sup>O) of tree-ring cellulose are a novel proxy of summer hydroclimate in monsoonal Asia. In central Japan, we collected 67 conifer wood samples, mainly <i>Chamaecyparis obtusa</i>, with ages encompassing the past 2,600 yr. The samples were taken from living old trees, excavated archeological wood, old architectural wood, and naturally buried logs. We analyzed stable isotope ratios of oxygen (δ<sup>18</sup>O) and hydrogen (δ<sup>2</sup>H) in tree-ring cellulose in these samples without using a pooling method, and constructed a statistically reliable tree-ring cellulose δ<sup>18</sup>O time-series for the past 2,500 yr. However, there were distinct age trends and level offsets in the δ<sup>18</sup>O record, and cellulose δ<sup>18</sup>O values showed a gradual decrease as an individual tree matures. This suggested it is difficult to establish a cellulose δ<sup>18</sup>O chronology for low-frequency signals by simple averaging of all the δ<sup>18</sup>O time-series data. However, there were opposite age trends in the cellulose δ<sup>2</sup>H, and δ<sup>2</sup>H gradually increased with tree age. There were clear positive correlations in the short periodicity variations between δ<sup>18</sup>O and δ<sup>2</sup>H, probably indicating a common climate signal. A comparison of the δ<sup>18</sup>O and δ<sup>2</sup>H time-series in individual trees with tree-ring width suggested that the opposite age trends of δ<sup>18</sup>O and δ<sup>2</sup>H are caused by temporal changes in the degree of post-photosynthetic isotope exchange with xylem water, accompanied by changes in stem growth rate (growth effect) that are influenced by human activity in the forests of central Japan. Based on the assumptions that cellulose δ<sup>18</sup>O and δ<sup>2</sup>H vary positively and negatively with constant proportional coefficients due to climate variations and the growth effect, respectively, we solved simultaneous equations for the climatological and physiological components of variations in tree-ring cellulose δ<sup>18</sup>O and δ<sup>2</sup>H in order to remove the age trend (growth effect). This enabled us to evaluate the climatic record from cellulose δ<sup>18</sup>O variations. The extracted climatological component in the cellulose δ<sup>18</sup>O for the past 2,600 yr in central Japan was well correlated with numerous instrumental, historical, and paleoclimatological records of past summer climate at various spatial and temporal scales. This indicates that integration of tree-ring cellulose δ<sup>18</sup>O and δ<sup>2</sup>H data is a promising method to reconstruct past summer climate variations on annual to millennial time-scales, irrespective of the growth affect. However, analytical and statistical methods need to be improved for further development of this climate proxy.</p>