To obtain the subject carbon fiber suitable for an organic solvent-based electric double layer capacitor, in which an electric double layer capacitor made by using the carbon fiber demonstrates higher service capacity than the conventional electric double layer capacitor.

This is a mesophase pitch-based activated carbon fiber having a BET specific surface area of 500m2/g or more. The carbon fiber is obtained by homogeneously mixing a mesophase pitch-based thermoset fiber or a mesophase pitch-based carbon fiber with an alkali metal compound in an amount of 0.5-5 times by weight, in which the carbon fiber is mildly carbonized at a temperature not higher than 800°C after thermosetting, and subsequently treating it by heating at a temperature of 550-900°C.