A slice management system and a slice management method that can dynamically adjust the amount of resources when the amount of resources is insufficient to create a slice. A storing unit of a NFVO stores slice utilization status information. In a BSS/OSS, an allocation determination unit determines to create a new slice or extend an existing slice, and when resources for creation of a new slice or extension of an existing slice are insufficient, a resource change determination unit determines a slice for reduction based on the slice utilization status information. A resource request unit of the NFVO reduces resources of the determined slice.