PURPOSE: To obtain a new fibrous composite metal hydroxide useful as a flame- retardant having a temperature for removing a water of crystallization adjusted to a proper region, almost free from preparation of hexagonal magnesium hydroxide as a by-product to reduce reinforcing effects of a resin and rubber, to obtain a composite metal basic salt of needle-like crystal useful for producing the fibrous composite metal hydroxide and to provide a method of producing the fibrous composite metal hydroxide.

CONSTITUTION: A fibrous composite metal hydroxide of formula (1) M2+xMg1-x(OH)2 (in the formula M2+ is at least one of Mn, Fe, Co, Ni, Cu and Zn; (x) is 0.005<(x)<0.5) has ≥ about 10 ratio of length/diameter, about 0.1-10μm average diameter and about 2-1,000μm average length. A composite metal basic salt of needle-like crystal of formula (5) M2+xMg1-x(OH)2-zA-z.mH2O (5) (in the formula M2+ is as shown for formula (1); A- is monofunctional anion; (x) is 0.005(x)<0.5; (z) is 0.2<(z)<1).