To provide a one-pack epoxy resin composition for underfill sealing having sufficient repairing properties and excellent reworkability of a wiring substrate and exhibiting performance-improving effects as compared with those of a conventional technique in productivity, solvent cleaning properties, connection durability etc.

The one-pack epoxy resin composition is used for the underfill sealing of a space between the wiring substrate and a semiconductor element electrically connected to the wiring substrate or a carrier substrate for holding the semiconductor element. The one-pack epoxy resin composition comprises an epoxy resin, an acid anhydride and a carboxylic acid represented by general formula (1): R1-O-CO-R2-COOH (wherein, R1 is a 1-20C alkyl group or a 1-20C fluorinated alkyl group which may contain an ether bond, or an ester bond or a 6-20C aromatic hydrocarbon group; and R2 represents a 1-20C chain or a cyclic bivalent hydrocarbon group).