To obtain stable and high conductivity carbon fiber, hydrocarbon is thermodecomposed in vapor phase into carbon fiber with such a crystal structure that carbon hexagonal net planes thereof are arranged substantially in parallel to fiber axis and in annular ring growth fashion around the fiber axis, and then the carbon fiber is pulverized into 10 .mu.m or less long carbon fiber. Since the carbon fiber is pulverized into relatively short length, when mixed with resin or rubber material without damaging the carbon crystal structure, it is possible to obtain stable and high conductivity resin or rubber material. Further, it is also possible to effectively infiltrate reactive substance to between the carbon hexagonal net planes to form further higher conductivity carbon intercalation compound.