Thermosetting resin composition

A method of using a cyclic carbodiimide compound (component B) including a cyclic structure having one carbodiimide group and represented by the following formula (B-i) in which first nitrogen and second nitrogen are bonded together by a bond group and having 8 to 50 atoms forming the cyclic structure as a curing agent for an epoxy resin (component A):

• (wherein Q is bivalent to tetravalent bond group which is an aliphatic group, alicyclic group, aromatic group, or a combination of these groups, and may contain a hetero atom or substituent).

A method of using a cyclic carbodiimide compound (component B) including a cyclic structure having one carbodiimide group and represented by the following formula (B-i) in which first nitrogen and second nitrogen are bonded together by a bond group and having 8 to 50 atoms forming the cyclic structure as a curing accelerator for an epoxy resin (component A):

• (wherein Q is bivalent to tetravalent bond group which is an aliphatic group, alicyclic group, aromatic group, or a combination of these groups and may contain a hetero atom or substituent).

A cured resin comprising a compound represented by the following formula (b-ii):

• (wherein Ar1 to Ar4 are each independently a phenylene group or naphthalene-diyl group which may be substituted by a substituent. W1, W2, X1, X2, Y1, Y2, and Z2 are each independently selected from a hydrogen atom and group represented by the following formula (b-iii), with the proviso that at least two out of W1, W2, X1, X2, Y1, Y2, Z1 and Z2 are groups represented by the following formula (b-iii))
• (wherein the wavy part represents an epoxy resin residue).