The Okhotsk Coast of Hokkaido is covered with drift ice in winter. Drift ice that rushes into Saroma Lagoon causes heavy damage to scallops and oysters, and aquacultural facilities. So, an ice boom that prevents drift ice from rushing into the lagoon was constructed. The ice force acting on this ice boom depends on the fluid force, and it is determined by the flow velocity and the area covered with ice floes. But the measured values of the ice force do not show the dependence on the flow velocity and the area covered with ice floes.In this paper the way how the ice force is acting on the ice boom is examined, and results obtained from model experiments on the arching of ice floes is reported.