Laboratory observations on young Japanese giant flying squirrel, Petaurista leucogenys, hand-reared after capture and field observations on the juvenile were carried out in order to clarify the development of locomotory ability and the change with the growth in the relationship between a mother and her offspring. 1. The eyes of the young opened at the age of about 30 days and they moved actively in the nest at about 40 days. Afterward, the young gradually showed tendencies to be nocturnal and to follow other individuals. The juvenile remained at the nesting tree for several days after the first emergence from the nest (～52 days); and began to extend its home range by following the mother. After the age of 70 days, it sometimes lodged at a separate nest from the mother's. 2. The young at 30 days was still unable to move on horizontal bars (diam.≦45 mm). The juvenile at 60 days could not climb up narrow vertical bars (diam.≦25 mm), and that at 90 days was almost adept in the quadrupedal locomotion as in the adult. 3. The juvenile made the first glide, preceded by the mother, several days after the first emergence. During a few weeks after emergence, it glided only a short distance and alighted occasionally on the ground. 4. The mother returned to the nest a few times a night to nurse her young. When the young accidentally fell to the ground, the mother retrieved it to the nest. Further, the mother protected and guided her juvenile out of the nest for some time after emergence. 5. The prolonged time required for the first emergence and the thorough care (protection and guidance) of the juvenile by the mother, which are regarded as characters of the behavioral development in this species, as compared with tree squirrels, seem to be adaptations for long time required to complete the gliding ability, which demands a complex coordination of muscle movement.