1. A computer comprising: a memory; and a processor being operable to manage private clouds and public clouds by using information stored in the memory, and to manage a migration of a set of an application and a data between the private clouds and the public clouds, the application running on at least one of a server of the private clouds or public clouds, the data to be read or written by the application and being stored in at least one of a storage system of the private clouds or public clouds; wherein the processor manages the migration of the set of the application and the data between the private clouds and the public clouds based on resource utilization of resources including at least one of the server or the storage system; wherein the processor is configured to select a set of application and data of private clouds, among a plurality of sets of application and data of private and public clouds, to be migrated, when the resource utilization of the private clouds exceeds a first threshold, so as to keep a high utilization ratio of the private clouds and not to exceed to the first threshold, regardless of a utilization ratio of the public clouds; wherein the first threshold is defined based on at least one of server usage rate or storage usage rate; and wherein when the resource utilization ratio of the private cloud exceeds the first threshold, the processor is configured to select for migration from private cloud to a public cloud, among the plurality of applications, an application which is more dynamic than other applications of the plurality of applications; wherein the application is more dynamic if at least one of following conditions is met: (i) the application has a standard deviation of resource utilization which is larger than the other applications, (ii) the application has a growth ratio of resource utilization which is higher than the other applications, and (iii) the application has a remaining period which is shorter than the other applications.