A flame retardant that is arranged to improve the cold resistance and the productivity of a composition, a flame-retardant composition, an insulated wire, a wiring harness, and a method for manufacturing the flame-retardant composition. The flame retardant includes a pulverized natural mineral which is mainly composed of magnesium hydroxide, wherein the pulverized natural mineral is surface-treated with an organic high polymer such as polyolefin. The amount of surface treatment is within a range of 0.1 to 10 mass %. The flame-retardant composition includes 30 to 250 parts by mass of the flame retardant with respect to 100 parts by mass of an organic high polymer such as polyolefin. The insulated wire includes a conductor and the flame-retardant composition which covers the conductor, and the wiring harness includes the insulated wire.