A stocker includes a storage shelf, an output-relay shelf, a first crane, an output shelf, a second crane, and a controller. The storage shelf has a plurality of storage spaces. The output-relay shelf has a plurality of first output-relay spaces. The output shelf has an output space. The controller is configured to drive the first crane to preferentially transfer a first wafer carrier stored in one of the storage spaces to an empty one of the first output-relay spaces according to a delivery command defining a high priority of the first wafer carrier, and configured to drive the second crane to preferentially transfer the first wafer carrier from the first output-relay space storing the first wafer carrier to the output space according to the delivery command if the output space is empty.