PURPOSE: To obtain a low-cost and high-speed magnetic disk device by modifying a part of firmware and data delivery circuit within an HDD.

CONSTITUTION: A requesing signal (REQ) synchronizing circuit 11 serves to match the timing of command delivery, that is, to perform synchronization, between a magnetic disk devices (HDD) 10a and 10b when it enters a command phase, and is interposed between a host computer (HOST) 9 and the HDDs 10a and 10b. The HOST 9, REQ signal synchronizing circuit 11 and HDDs 10a and 10b are connected by a small-sized computer system interface (SCS1) bus 12. Accordingly, the HDDs 10a and 10b can be synchronized with the REQ only at the time of commanding phase, and both of them can recognize the command. Thus, the magnetic disk device capable of duplicating a throughput at the time of READ compared with a single HDD can be easily obtained at low cost.