The object of the present invention is to provide a fiber sheet or a molded fiber sheet having excellent flame retardancy A flame retardant fiber sheet containing a polyammonium phosphate with average degree of polymerization in the range of between 10 and 40 is provided in the present invention. The polyammonium phosphate is insoluble or sparingly soluble in water so that said polyammonium phosphate gives the fiber sheet excellent flame retardancy with water resistance and durability, and is also inexpensive. The molded fiber sheet is highly flame retardant and harmless, so that the molded fiber sheet is useful for automobile and building interiors.