A study was conducted in Lake Hinuma, northern central Japan, in order to elucidate the role of a brackish water lake in the early life history of amphidromous ayu, Plecoglossus altivelis (Plecoglossidae). Descriptions of amphidromous P. altivelis larvae based on wild specimens are also given. Larvae first appeared in Lake Hinuma in October and remained inside the lake until December. During this period, continuous recruitment of newly hatched yolk-sac larvae was observed. The size of larvae found in the system ranged from 5.2–32.1 mm SL, representing the entire larval period of the species, from the newly hatched yolk-sac larvae to metamorphosed juveniles. Larvae emigrated out of the Lake Hinuma system to the coastal waters after the surface water temperature of the lake dropped below 6° C and became lower than that of adjacent coastal waters. It is concluded that brackish water lakes play an important role in the early life history of amphidromous P. altivelis, as a temporary nursery ground.