To delete input operation from an external storage device by buffering by providing a common file control means for the other host computer when the host computers share a file and maintaining the consistency to data on the common file.

The host computers 1 and 2 are connected by a fast communication bus 70 and shares a magnetic disk drive 8. When the host computer 1 updates data 82 in the common file 81 on the magnetic disk drive 8 in this state, the host computer 1 turns off a buffer presence flag 66 of a buffer control table 65 regarding a buffer area 67 for data 82 of the host computer 2 and sends a request to invalidates the buffer area 67 to the host computer 2 before performing a writing process to the common file 81. The host computer 1 after being informed of success by the host computer 2 and confirming that writes the data to the magnetic disk drive 8.