A liquid thermosetting epoxy resin composition contains a base resin in combination with a curing agent and a curing accelerator or with a curing catalyst. The base resin includes a cycloaliphatic epoxy compound having at least one alicyclic skeleton and two or more epoxy groups per molecule, and a polyol oligomer having two or more terminal hydroxyl groups. An optical semiconductor device includes an optical semiconductor element sealed by using the liquid thermosetting epoxy resin composition. The composition yields a cured resinous product which is free from curing failure, is optically homogenous, has a low elastic modulus in bending, a high bending strength, a high glass transition temperature, a high optical transparency and is useful for optical semiconductors.