The present invention provides an one-pack thermosetting type epoxy resin composition which is useful as an underfilling material used when a flip chip or a semiconductor package comprised of a semiconductor element held on a carrier base being mounted onto a wiring substrate; which can omit a fluxing process as adopted for improvement of the bonding force of bumps or solder balls particularly at said mounting and exhibits a good voidless property even at a reflow temperature; and which can be also applied as an adhesive, a paint, a coating material, a sealing material, or the like.The one-pack thermosetting type epoxy resin composition of the present invention comprises as essential ingredients, a liquid epoxy resin and a carboxylic acid having two or more carboxylic groups in molecule as a curing agent.