The invention provides a novel epoxy-based resin composition which is storable as ready-mixed in one package in a sealed condition for a long time of period and yet is curable when exposed to the atmospheric air without heating by the action of the moisture contained in the atmospheric air to exhibit very strong adhesive bonding to various kinds of substrate materials. The inventive composition comprises (a) an epoxy compound having at least two epoxy groups in a molecule, (b) a curing agent for the epoxy compound, and (c) an organosilicon compound, in particular, an organosilane compound or an organopolysiloxane, having, in a molecule, at least one group represented by the general formula ##STR1## where R.sup.1 and R.sup.2 are each a hydrogen atom or a monovalent hydrocarbon group and R.sup.3 is a divalent hydrocarbon group, directly bonded to the silicon atom. The component (c) reacts with the component (b) when blended together to mask the activity of the curing agent but readily regenerate an active curing agent when reacted by the moisture in the atmospheric air even at room temperature effecting curing of the epoxy resin composition.