1. A computer comprising: a memory; and a processor being operable to manage two types of clouds, which are private 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 a first type of clouds which are private clouds and a second type of clouds which are public clouds, the application running on a server of the first type of clouds, the data to be read or written by the application and being stored in a storage system of the first type of clouds; wherein the processor manages the migration of the set of the application and the data between the first type of clouds and the second type of clouds based on resource utilization of resources including at least one of the server or the storage system so as to keep a high utilization ratio of the first type of clouds as compared to the second type of clouds; wherein the processor is configured to migrate the set of the application and the data, from the first type of clouds including one or more private clouds to the second type of clouds including one or more public clouds, when the resource utilization of the first type of clouds exceeds a first threshold, so as to keep a high utilization ratio of the first type of clouds until the first threshold is exceeded, without regard to a utilization ratio of the second type of clouds; wherein the first threshold is defined based on at least one of server usage rate or storage usage rate; wherein the processor is configured to monitor a plurality of applications on one type of clouds as a migration source for the migration of the set of application and the data; wherein the processor is configured to select for migration from a 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 when the resource utilization of the private cloud exceeds the first threshold; and wherein an application is more dynamic if at least one of the 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.