1. A network design apparatus comprising: a memory; and a processor coupled to the memory, wherein the processor executes a process including: calculating an allocation pattern not requiring cancellation of a connection request from among a plurality of allocation candidates, when the connection request transmitted/received between nodes on a network is to be allocated to a slot that constructs a link on the network; determining a change procedure of the connection request in order to change an allocation pattern provided before the network is re-optimized to the allocation pattern calculated at the calculating; and outputting the allocation pattern calculated at the calculating as an allocation pattern after the network is re-optimized, along with the change procedure determined at the determining.