As a fundamental study to clarify the agglomeration and coalescence of alumina inclusions in molten steel from the viewpoint of interfacial chemical interactions, it has been experimentally verified for the first time that significant agglomeration force is exerted between alumina particles in aluminum deoxidized molten steel by using a newly established experimental method. In this method, the agglomeration force exerted between alumina particles in molten steel is directly measured separately from the effect of molten steel flow. In addition, it has been quantitatively demonstrated that the contact angles measured between aluminum deoxidized molten steel and an alumina plate are larger than those between the molten iron-oxygen alloy and the alumina plate, which have already been measured by other researchers. Moreover, it has also been indicated by analyzing the actual measurement values of agglomeration force with an interaction model taking contact angles and interfacial properties into consideration that the agglomeration force between the alumina particles in aluminum deoxidized molten steel derives not from the van der Waals force but from the cavity bridge force occurring due to molten steel, which is unlikely to wet the alumina particles. Meanwhile, it has been assumed that the agglomeration force on spherical alumina inclusions in aluminum deoxidized molten steel calculated on the basis of the interaction model according to the cavity bridge force is greater than the buoyant force and drag force, and the alumina inclusions once coming into contact are therefore not prone to be simply dissociated even under molten steel flow. Thus, they maintain the agglomeration state and are subsequently sintered and form comparatively solidly bonded alumina clusters.