PURPOSE: To reduce the burden of software on a host device-side by outputting a selection signal to a control circuit without recognizing which control circuit is the object of first access on the host device-side.

CONSTITUTION: When a change over switch 13 is set on an ST 506-side, an address signal and the like are inputted to a disk switching control circuit 14 if a host selects a first hard disk. The disk 1 signal of H from the circuit 14 is inverted in an invertor 14a and is outputted to a selection terminal in an ST 506 control circuit 1. The circuit 1 is selected at that time and an IDE control circuit 1A inverts an L signal again whereby the signal is inputted to a selection terminal as the H signal. Thus the circuit 1A is not selected and it is detached from a system bus 2. Consequently, a command from the host is received only by the circuit 1 and it executes the command. When the host selects the second hard disk in such a state, the circuit 14 latches the disk 1 signal output to L. Consequently, the command from the host is received only by the circuit 1A and it executes the command.