Relationship between stem diameter at breast height (DBH) and tree height (H) among individuals at a particular point in time does not agree with the time trajectory of a DBH–H relationship of an individual tree. We examined long-term changes in the among-tree relationship by considering time trajectories of DBH–H relationship of individuals using a 20-year record of a Chamaecyparis obtusa population. Vigorously growing trees showed almost linear DBH–H trajectories throughout. Even in trees with weak DBH growth, their DBH–H trajectories also exhibited somewhat linear patterns, but with steeper slopes. Their DBH became asymptotic against H (i.e., H growth was somewhat maintained while DBH hardly increased) just before they died of competition. In addition, the movement of (DBH, H) data point along a trajectory with time was slower in the weakly growing trees than did vigorously growing trees. As a result, the among-tree DBH–H relationship at a point in time exhibited a pattern that H was asymptotic against DBH. From separate analyses, we found that within the same individual in a given year, stem diameter growth rates at different heights along a stem were almost equal in the below-crown part of a stem. Hence, DBH growth rate represented diameter growth rates of only the below-crown part of a stem.