Distribution of the sea urchin Hemicentrotus pulcherrimus was monitored monthly or bimonthly from April 1998 to May 1999 at a 100 m² permanent site in intertidal to subtidal zones at depths of up to 2.8 m in Onagawa Bay, northern Honshu, Japan. Factors causing seasonal demographic changes were analyzed from vertical algal vegetation, sea urchin size, gonad development, and gut content. The highest density of H. pulcherrimus was found on algal turf on boulders at the depth of 0–0.3 m. Migration of adult sea urchins on crustose corallines at the depth of 0.3–2.8 m to the algal turf and subsequent inhabitation in more suitable boulder habitat are suggested from November to March, when Acrosorium polyneurum, which is their main food, grew dominantly. As the high gut content of crustose coralline of the sea urchins on crustose corallines resulted in significantly lower gonad indices, it is likely that migration is caused by their high foraging activity for A. polyneurum to ensure gonad development. The growth of the sea urchins on algal turf was ensured by Chondrus spp. as main food.