PURPOSE: To produce stably and inexpensively a solid amino resin used for amino resin molding materials, by dehydrating and condensing a resin solution prepared by cocondensing melamine, formaldehyde and a cyclic lactam with a urea-formaldehyde resin precondensate.

CONSTITUTION: Urea and formaldehyde are reacted in a mol ratio of 1/1.3W1/2.0 in the presence of an alkaline catalyst, yielding a urea-formaldehyde resin precondensate. The resin precondensate is cocondensed with melamine in an amount to provide a mol ratio to urea, of 1/15W1/3, formaldehyde in an amount to provide a mol ratio to melamine, of 1.3W2.7, and a cyclic lactam in an amount to provide a mol ratio to the sum of mol numbers of urea and melamine, of 0.05W0.6, said cyclic lactam being represented by the formula, wherein R is H or alkyl, and n is an integer of 1W5, yileding a resin solution. Then, the resin solution is condensed by dehydration, yielding a solid amino resin.