To realize a server duplex system for a highly reliable LAN at a low cost.

A working server 1A and a standby server 1B are connected to HDDs 5 (51 to 56) through a common SCSI bus 4 and an SCSI bus control device 3 is inserted between the servers 1A, 1B and the HDDs 5. One server allowed to write data in each HDD 5 is set up in a setting switch part 32. In this case, the server 1A has the writing right of the HDDs 51 to 53 and the server 1B has the writing right of the HDDs 54 to 56. When the server 1A (1B) generates a writing command to the HDD 5 whose writing right is not owned, a bus switching control part 31A (31B) switches bus changing switches SW1A, SW2A (SW1A, SW2B) to the (b) side, sends a signal for stopping the writing access to the server and the HDD, and after checking the stop, the bus changing switches SW1A, SW2A (SW1B, SW2B) are restored. The standby server 1B copies the contents of the HDDs 51 to 53 written by the server 1A to the HDDs 54 to 56 in bus idle time.