The adaptation for glide in the Japanese giant flying squirrel, Petaurista leucogenys, was investigated from the view point of relative growth, applying the allometry formula y=bx^α, together with a few field observations on gliding postures. The results obtained are summarized as follows: 1. The body weight and the aspect ratio of the flying squirrel when it became able to glide were no more than 42% and 69% of those of the adult, respectively. It took the animal more than a month after leaving nest to achieve a full gliding ability. 2. The body weight showed a monophasic positive allometry (α=2.83) against the head and body length. The tail length and the hindfoot length against the head and body length exhibited a positive allometry before leaving nest, and a negative allometry after leaving nest. Moreover, the growth rates of forelimb, hindlimb, and forelimb+styliform cartilage after leaving nest were slightly inferior to that of the head and body. 3. The length of long bones of the limbs showed a diphasic allometry against the condylobasal length of skull, with an inflexible point at the time of leaving nest; the distal bones grew more quickly before leaving nest than did the proximal ones, although the reverse was the case after leaving nest. 4. The stylirorm cartilage was still weak when the animal emerged from the nest. Afterward, however, it became strong and played an important role in the improvement of aerodynamic performances such as the aspect ratio and wing-tip shape of the flying membranes. 5. In the case of the flying squirrel, unlike bats, the wing loading did not decrease significantly according to its growth. Decrease of the ponderal ratio, reinforcement of the styliform cartilage and morphological changes of the patagia were involved in lowering the wing loading. On the contrary, the relatively slow growth of the limbs, as compared with that of the trunk, effected increase of the wing loading. The antagonism of these factors seemed borne a peculiar growth phase of the patagia. 6. The flying squirrel depending mainly upon glide for its locomotion acquired a growth pattern suitable for gliding, together with the growth characters relating to adaptation for the arboreal life.