Spawning season of Strongylocentrotus intermedius in Hokkaido is classified into the 3 types, Sea of Japan, Funka Bay, Sea of Okhotsk, and Pacific Ocean. To clarify the type in Pacific Ocean, northern Honshu, we calculated seasonal gonad indices (gonad wet weight/body weight × 100) monthly and plotted the gonad development process from histological observation of Strongylocentrotus intermedius collected at depths of 0.5–1.0 m in Hirota Bay, Iwate Prefecture, from September 2005 to November 2006. The gonad indices showed a seasonal cycle: they reached peaks of 20.7–23.5 from September to October, when the gonads were in the maturation stage. They fell sharply to 1.9–3.4 in November, corresponding to the spent stage. They remained low until February, when the gonads progressed to recovering stage. They rose again with water temperature in March; the gonads were in the growing stage during March to May. In June, premature gonads appeared, and dominated in September. Spawning was vigorous in October as the water temperature fell from 20.0°C to 13.0°C, coinciding with that in the Sea of Japan off Hokkaido.