The Japanese clawed salamanders (Onychodactylus japonicus) have a long larval period in streams and after metamorphosis they begin a terrestrial life near the stream. Different feeding rates between the stream and terrestrial habitats may be an important factor affecting their life history traits, particularly the body size and seasonal timing at metamorphosis. In this study, we estimated prey availability in streams, prey eaten by larval salamanders, prey availability on the forest ground, larval growth pattern, and body size at metamorphosis, based on the seasonal surveys at the mountainous stream in Tochigi from 2015 to 2016. As a result, larvae stayed for at least 3 years in streams and left streams in September. Larger larvae tended to eat larger prey. The density of available prey in streams decreased in spring and autumn. Larger prey was more available in spring to early summer. However, prey size distributions available in streams are generally skewed smaller than those available on the forest ground. Finally, we discuss the optimal body size and seasonal timing at metamorphosis for this salamander in the concept of a complex life cycle in which the predator shift the habitats to maximize the lifetime growth rate.