A vertical multi-stage fluidized bed apparatus including a plurality of horizontal perforated partitioning plates disposed therein so as to partition the apparatus is provided, wherein an upper horizontal perforated partitioning plate is set to have a larger aperture rate than a lower horizontal perforated partitioning plate. From a lower part of the apparatus, feed carbon and fluidizing gas are continuously supplied so as to provide a gas superficial velocity in the fluidized bed which is 2 - 4 times a minimum fluidizing velocity of the feed carbon, thereby subjecting the feed carbon to fluidization with the fluidizing gas and activation with steam at 750 - 950°C simultaneously to discharge activated carbon continuously from an upper part of the apparatus. As a result, activated carbon of even a high degree of activation is produced at a high yield comparable to that obtained in a batchwise operation.