A stocker may include a load port which a cassette for receiving wafers is either loaded on or unloaded from, an aligner configured to align the wafers and to confirm identification codes of the wafers, a plurality of shelves each having slots for receiving the wafers, a first transfer robot having a first robot arm for transferring the wafers between the load port and the aligner and a second transfer robot having a second robot arm for transferring the wafers between the aligner and the shelves.