To understand the age structure of branches influencing crown development by the elongation of branch axes in the upper crown of Abies sachalinensis canopy trees, a relationship between the foliage mass index and branch-axis length for a 13-year growing season was examined by a linear mixed model using the past main-stem and branch-axis elongation record. The record was investigated for 10 canopy trees with sunlit branches from the crown top to at least 20-year-old branches in a cold temperate forest near Nakayama Pass in southwestern Hokkaido, Japan. For each year during the 13-year period, the effect of foliage mass on branch-axis elongation was compared between five branch age groups in a range of 1 to 20 years. To determine the foliage mass index for each branch age group, a portion of the main stem’s volume was calculated using an allometric relationship. The branch-axis elongation records for 3-, 4-, and 5-year-old branches were also investigated for each year. On each branch-axis elongation, the foliage mass index of the 9- to 12-year-old branch group indicated a greater effect than that of the other branch groups, suggesting a relatively greater contribution of the foliage mass of nearly 10-year-old branches to crown development in the sunlit crown.