To obtain a cured product having high segment packing to suppress micro-Brownian motion and exhibiting excellent heat-resistance by curing a specific biphenol resin containing less than a specific amount of unreacted biphenol.

This biphenol resin is produced by condensing a biphenol compound having one or more ≤4C alkyl groups as substituents on an aromatic ring or an unsubstituted biphenol compound such as 4,4-biphenol with an aldehyde or condensing the above biphenol compound with a non-rigid component expressed by formula I (R1 is OH, H, a ≤9C alkyl or an aromatic residue) such as phenol and/or a non-rigid component expressed by formula II (B1 and B2 are each 1,4-phenylene, 2,6-naphthalene, etc.; Y1 and Y2 are each a ≤10C organic residue, O, COO, etc.; R2 is OH or NH2; (m) is 0-1; (n) is 0-3) such as bisphenol A and an aldehyde. The biphenol resin containing ≤20wt.% of unreacted biphenol in the resin is cured.