1. A file-sharing system comprising: a first file server ( 200 , 1800 ) including a first scan processing section ( 290 , 1810 ) configured to scan data included in a file to generate predetermined information, the first file server ( 200 , 1800 ) being configured to provide a file to a client terminal ( 100 ) based on a virtual file system ( 360 ) that is generated by virtually merging a first file system ( 370 , 380 ) and a second file system ( 490 ); and a second file server ( 400 , 1900 ) including a second scan processing section ( 460 , 1910 ) and the second file system ( 490 ), the second scan processing section ( 460 , 1910 ) being configured to scan data included in a file to generate predetermined information, wherein: the first scan processing section ( 290 , 1810 ), in response to a first scan processing request for files included in the first and second file systems ( 370 , 380 , 490 ) issued by a management terminal ( 110 ), executes scan processing to the files included in the first file system ( 370 , 380 ) based on the virtual file system ( 360 ), and sends a second scan processing request to the second file server ( 400 , 1900 ) to execute scan processing to the files included in the second file system ( 490 ), the second scan processing section ( 400 , 1910 ), in response to the second scan processing request received, executes scan processing to the files included in the second file system and sends the scan processing result to the first file server ( 200 , 1800 ), the first file server ( 200 , 1800 ) merges the scan processing result obtained with the first scan processing section ( 290 , 1810 ) and the scan processing result obtained with the second scan processing section ( 460 , 1910 ), and provides the merged result to the management terminal ( 110 ), the first file server is a file server ( 200 , 1800 ) connected to the client terminal ( 100 ) and the management terminal ( 110 ) via a network ( 120 ), the second file server is a cloud computing environment ( 400 , 1900 ) connected to the first file server ( 200 , 1800 ) via a network ( 130 ), the first file system is an upper-level real file system ( 370 , 380 ) provided by a disk array system ( 300 ) connected to the first file server ( 200 , 1800 ), the second file system is a lower-level real file system ( 490 ) provided by the cloud computing environment ( 400 , 1900 ), the virtual file system ( 360 ) constitutes a hierarchical file system with the upper-level real file system ( 370 , 380 ) and the lower-level real file system ( 490 ), the first file server ( 200 , 1800 ) further includes an object management table ( 800 ) that manages a correspondence relationship between the virtual file system ( 360 ) and a storage location of a real file or directory, and a batch processing flag ( 860 ) indicating that all files included in a single directory have been migrated to the lower-level real file system ( 490 ), and also includes an inter-tier migration processing section ( 540 ) configured to migrate a file from the upper-level real file system ( 370 , 380 ) to the lower-level real file system ( 490 ) in accordance with the conditions described in an inter-tier migration policy ( 550 ) that is a policy for migrating files between file systems of different tier levels, the first scan processing section ( 290 , 1810 ) refers to the object management table ( 800 ) for a file and a directory corresponding to the first scan processing request, and sends the second scan processing request to the second file server ( 400 , 1900 ) to execute scan processing to all files included in a directory whose batch processing flag ( 860 ) is ON, and executes by itself scan processing to files that are included in a directory whose batch processing flag ( 860 ) is OFF and are included in the second file system ( 490 ), instead of instructing the second scan processing section ( 460 , 1910 ) to execute scan processing to such files, and the second scan processing section ( 460 , 1910 ), in response to the second scan processing request received, executes scan processing to the files included in the directory whose batch processing flag ( 860 ) is ON, and sends the scan processing result to the first file server ( 200 , 1800 ).