An activated and heat-treated product of a pitch fiber (A) is combined with an activated and heat-treated product of a precursor fiber of carbon fiber (B) having a larger elongation and a larger shrinkage during activation treatment thereof than those of the pitch fiber (A) to provide an activated carbon fiber structure. The activated carbon fiber structure is produced by subjecting the pitch fiber (A) and the precursor fiber of carbon fiber (B) to an activation treatment before or after the fibers (A) and (B) are formed into a configuration corresponding to a fiber structure through mixing or laminating.