To obtain a formaldehyde-free flame retardant compsn. for paper which is excellent in heat resistance and does not degrade paper by mixing a guanidine salt with magnesium sulfate in a specified ratio.

This compsn. is a mixture of 90-40 wt.% guanidine salt (A) and 10-60 wt.% magnesium sulfate (B). A reaction mixture of dicyandiamide with ammonium sulfamate and/or ammonium sulfate, and guanidine sulfamate and/or guanidine sulfate are each esp. pref. as the guanidine salt. Pref., the compsn. further contains a urea compd., esp. urea, usually in an amt. of 1-20 pts.wt. based on 100 pts.wt. mixture of ingredients A and B. Pref., the compsn. is applied in the form of a water soln. when used for treating paper, and the concn. of the compsn. in the water soln. is pref. 20-40 wt.%.