PURPOSE: The titled novel resin to be melted and to be dissolved, having a high average molecular weight, by condensing a phenol with a carbonyl compound in a specific equivalent ratio at a middle temperature in a short time, subjecting it to a high polymer reaction under specific conditions.

CONSTITUTION: A mixture of (A) a phenol and (B) a carbonyl compound satisfying respectively the equation shown by the formulaI(fP and fF are the average functionality of the component A and the component B respectively), having an equivalent ratio of the component A to component B satisfying the equation shown by the formula II is first condensed at a middle temperature in a short time, and the condensate is subjected to high polymer reaction at high temperature for a long time until the viscosity of the system is extremely increased while volatile components are removed under neutral or weakly acidic conditions, to give the desired resin having the unreacted component A as less as possible, having a number-average molecular weight MN calculated by removing the unreacted component A, and a ratio of a weight-average molecular weight MW to MN of ≥4 calculated by removing the unreacted component A.

EFFECT: The resin having improved physical and chemical properties is obtained economically in high yield.