Disclosed is a liquid thermosetting epoxy resin composition including a base resin, a curing agent (D), and curing accelerator (E) or including a base resin and a curing catalyst (D′), each base resin including an alicyclic epoxy compound (A) having at least one alicyclic skeleton and two or more epoxy groups per molecule; a polyester polyol and/or a polycarbonate polyol (B), each polyol having two or more terminal hydroxyl groups; and a phosphorous ester (C). The liquid thermosetting epoxy resin composition exhibits excellent viscosity stability and can give a cured article which is homogeneous even when having a large thickness, does not crack under heat cycle conditions, and is highly thermally stable and satisfactorily optically transparent.