An epoxy resin composition includes: an epoxy resin; a curing agent; a curing accelerator; and an inorganic filler, wherein the curing accelerator includes a 4-valent ammonium salt or a 4-valent phosphonium salt represented by Formula 1, wherein A1 is nitrogen or phosphorus; R1, R2, R3 and R4 are each independently a substituted or unsubstituted C1 to C30 hydrocarbon group, or a substituted or unsubstituted C1 to C30 hydrocarbon group including a hetero atom; X1, X2, X3, X4, X5 and X6 are each independently an oxygen atom (O), a sulfur atom (S), or NH; and Y1, Y2 and Y3 are each independently a substituted or unsubstituted C1 to C30 hydrocarbon group, or a substituted or unsubstituted C1 to C30 hydrocarbon group including a hetero atom.