The molting growth pattern of the Japanese mitten crab Eriocheir japonica (de Haan, 1835) was investigated under laboratoryreared conditions. Crabs were individually reared in freshwater for 5 years in a constant temperature room at 23–25°C. The age after metamorphosis and instar number were recorded for each molt, and the intermolt period was calculated for each crab. Carapace width (CW) was measured and the percentage molt increment was calculated. Eleven crabs reached a CW > 10 mm. Each growth curve (ageCW relationship) had two phases in the juvenile stage. Growth rate gradually decreased in the younger phase before changing to nearly constant in the older phase. During the younger phase, the percentage molt increment decreased from 25.0%–38.1% to 6.0%–26.2% (ca. 2–10 mm CW, 1st to 11th instars), and the intermolt period increased from 4–9 days to 40–300 days (ca. 2–20 mm CW, first to thirteenth instars). During the older phase, both parameters became broadly flat but showed marked fluctuations. Crabs reached minimum adult size (ca. 35 mm CW) or the adult stage at 2–4 years after metamorphosis. The instar numbers required by E. japonica to reaching maturity was more than that required by other brachyuran species. The adult stage appeared after the twentieth instar in females, and crabs reached their minimum adult size during the sixteenth to twenty-first instars. After puberty, one female continued molting growth. This suggests that adults could live longer and grow larger in size if they remain in a freshwater environment.