Marine algal succession on two coralline flats after removal of sea urchins was studied at Yaoi and Rokujo site in Suttsu Bay on the Japan Sea coast of Hokkaido from October 1990 to July 1993. Attaching diatoms, small annual algae such as Ulva pertusa and Polysiphonia morrowii, large annual algae such as Desmarestia viridis and Undaria pinnatifida, and small perennial algae such as Dictyopteris divaricata invaded to the coralline flats in order. They were followed by the large perennial alga Sargassum confusum which was a species endemic in the Japan Sea at Yaoi site. The small annual alga Ulva pertusa dominated in an early phase of algal succession, and its' dominance continued at Rokujo site due to the influences of an inflow of freshwater and sand from Shubuto River.