Abstract: A pitch-type fiber convertible to an activated carbon fiber is infusiblized with an infusiblizing gas and the infusiblized fiber is activated with an activating gas, such as steam, to produce an activated carbon fiber. The waste gas from the infusiblizing step is utilized in a combusting step. The combustible component of the waste gas from the activating step is separated out of the waste gas from the activating step and combusted in the combusting step along with the waste gas from the infusiblizing step. The resulting heat from the combusting step is utilized to preheat the infusiblizing gas and the activating gas and the preheated gases are utilized in the infusiblizing step and the activating step, respectively. This arrangement allows the infusiblizing temperature and the activating temperature to be accurately controlled. Thus, an activated carbon fiber is manufactured continuously without being negatively affected by waste gases from the infusiblizing and activating steps.