Effects of timing of overwintering larval-cocoon formation, constant temperature, and soil moisture on postdiapause development of Carposina sasakii Matsumura were examined. Also, a forecasting model of its spring emergence was constructed based on degree-days. The development time from overwintering larva to adult emergence was not significantly different among soil moistures in the range of 10–20%, but it was different according to the timing of larval-cocoon formation in the previous year. The weight of cocoons containing an overwintering larva was less for cocoons formed after 6 October than cocoons formed before 15 September. There was a positive correlation ( r = 0.47) between the overwintering larval-cocoon weight and its development time to adult. Approximately 3–4% of C. sasakii showed a prolonged diapause spending 2 yr as larval-cocoons in the soil. A greater proportion (18.4%) of heavier cocoons (>0.040 g) remained in diapause in the current year compared with normal weighed cocoons (0.022–0.040 g) (1.6%) or lighter cocoons (<0.022 g) (0%). The adult spring emergence showed a two-peak emergence pattern in which the first major peak occurred in late June and the second smaller peak in late July. A bimodal distribution model was developed to describe this emergence pattern. The bimodal model predicted more accurately C. sasakii spring emergence times than the Weibull model.