To obtain a curable resin having high thermal and mechanical properties and excellent in durability, chemical resistance, and compatibility with other resins by incorporating a polymer having cyclic structural units and reactive groups.

This resin comprises at least one polymer represented by the formula. In the formula, A to F each is a monomer unit as a component of the polymer backbone and may be arranged in any sequence, and (a) to (f) are the amounts (wt.%) of the monomer units A to F, respectively, based on the total amount of the monomer units A to F. The number-average mol.wt. in terms of standard PS of the polymer is 1,000-5,000,000. 'A' is a unit derived from a cyclic olefin monomer, B is one derived from a cyclic conjugated diene monomer, C is one derived from a chain conjugated diene monomer, D is one derived from a vinylaromatic monomer, E is one derived from a polar monomer, and F is an ethylene or α-olefin monomer unit. Q1 to Q6 each is a reactive group. 'A' to F satisfy the relationships a+b+c+d+e+f=100, 0≤(a, b)≤100, 0≤(c, d, e, f)<100, and a+b≠0.