PURPOSE: To form a film on the surface of acicular particle having a large surface area by introducing a reactive gas consisting of metal carbonyl into a rotary reaction vessel and causing decomposing reaction.

CONSTITUTION: A reaction furnace 2 has a door 3 and a drum type second reaction furnace 4 which rotates at a rate of 5W30rpm is provided within such reaction furnace. The inside pressure of first and second reaction vessels is adjusted to 0.01W10 torr by a needle valve 16. The reaction gas introduced from the doping system or the carrier gas, in addition to such gas, reaches the second reaction furnace. Thereby, a product of reaction grows or a film is formed on the surface of acicular magnetic powder 1. The reaction gas and carrier gas reaches a vacuum pump 10 through an exhaustion port 5. The acicular magnetic powder 1 is heated by a heating board 7 and microwave 9. In the doping system 20, the hydrogen carbonyl 15, iron carbonyl 14, nickel carbonyl 13 and cobalt carbonyl 12 are supplied and these are supplied to each port through valves.