To provide a curing agent for epoxy resins which has a low melt viscosity, excellent low stress properties, low hygroscopicity and an epoxy resin composition with excellent moldability, and a semiconductor sealing composition which use this curing agent.

The curing agent for epoxy resins comprises (a) 30-99 pts.wt. modified phenolic resin to be obtained by polycondensing a heavy oil or a pitch with a phenol and an aldehyde compound in the presence of an acid catalyst and (b) 1-70 pts.wt. thiodiphenolic compound to be represented by formula (wherein R1 may be the same or different from each other and is a 1-12C alkyl group, a substituted or unsubstituted phenyl group, a substituted or unsubstituted aralkyl group or an alkoxy group; and p may be the same or different from each other and is an integer of 0-3); and (c) 0-100 pts.wt. polyhydric phenolic compound having a structure other than those of components (a) and (b). The composition and the semiconductor sealing composition use this curing agent.