PURPOSE: To obtain the present co-condensate free from change of color by heat, reduction in water solubility, etc., in addition as a heat-resistant enhancer to a flame-retardant for cellulose, comprising dicyandiamide, salt of sulfamic acid and guanidine (derivative) and formaldehyde.

CONSTITUTION: The aimed co-condensate obtained by condensing (A) 1mol. dicyandiamide with (B) 0.01W0.2mol. salt of sulfamic acid and guanidine (derivative) and (C) 0.5W3mol. formaldehyde. The condensate is obtained by reacting the components A, B and C in the presence of a small amount of water required to promote the reaction at 40W90°C and cooling the reaction mixture. Paraformaldehyde is preferable as a feed source for the component C. The amount of water used is preferably 2W10wt.% based on total amount of the components AWC.