TECHNOLOGY FOCUS - TEXTILES AND PAPER - Preferred Method: The mesophase pitch is spun at 300-380 degrees Celsius, to obtain a pitch fiber. The obtained pitch fiber is subjected to infusible process at 250-350 degrees Celsius in oxygen-containing gas atmosphere, to obtain an infusible fiber. The obtained infusible fiber is carbonized at 500-900 degrees Celsius in inert-gas atmosphere, to obtain a carbon fiber. The obtained carbon fiber is ground, to obtain carbon fiber ground material. The carbon fiber ground material is mixed with an alkali metal compound in a weight ratio of 1.5-2.5, and maintained at 350-500 degrees Celsius in an inert-gas atmosphere for 2-4 hours. Then, the temperature is raised to 700-800 degrees Celsius with a temperature increase rate of 300 degrees Celsius or more and maintained for 2-4 hours.    TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Properties: The mesophase pitch has viscosity of 50-50000 cp at 300-380 degrees C, softening point of 250-320 degrees C, mesophase content of 70 % or more and metal-impurity content of 1000 wt.ppm or less. The pitch fiber has diameter of 5-20 micrometers. The infusible fiber has infusible degree of 20 wt.% or less. The carbon fiber has true density of 1.3-1.8 g/cm3, face distance of 0.345-0.365 nm when measured by X-ray diffraction, and carbon to hydrogen (C/H) atomic ratio of 2-8. The carbon fiber ground material has mean particle diameter of 1-100 micrometers when measured by laser diffraction method, and metal-impurity content of 1000 wt.ppm or less. EXAMPLE - A mesophase pitch was spun at 350 degrees Celsius, to obtain non-woven-fabric-like pitch fiber having diameter of 12 micrometers. The obtained pitch fiber was subjected to infusible process at 320 degrees Celsius for 0.5 hours, to obtain infusible fiber having infusible degree of 6 wt.%. The obtained infusible fiber was carbonized at 800 degrees Celsius in nitrogen gas atmosphere for 1 hour, to obtain carbon fiber having true density of 1.55 g/cm3, face distance of 0.355 nm and carbon to hydrogen atomic ratio of 4. The obtained carbon fiber was ground, to obtain carbon fiber ground material. The obtained carbon fiber ground fiber and potassium hydroxide were mixed in a weight ratio of 1:2 at 400 degrees Celsius in nitrogen gas atmosphere for 3 hours, and further reacted at 800 degrees Celsius for 2 hours. The processed fiber was washed using warm water and hydrochloric acid and dried at 110 degrees Celsius for 24 hours using air, to obtain activated carbon fiber. The obtained activated carbon fiber had BET specific surface area of 800 m2/g, pore diameter of 0.8 nm and pore volume of 0.35 ml/g, and provided an electrical-double-layer capacitor with excellent electrostatic capacitance.