PURPOSE: To eliminate the need of the lock release procedure of a host computer by monitoring an SCSI(Small Computer Systems Interface) bus at the external part of the host computer, informing the host computer of a locking state when it is detected and releasing locking.

CONSTITUTION: When a state transition monitor circuit 1 detects the state transition of the SCSI bus 101, it transmits a reset signal to a timer counter 2. The counter 2 restores a discrete value to zero by the reset signal and starts counting. When the SCSI bus 101 is not state-transited, an overflow signal is transmitted to a notice circuit 3 when the counter 2 exceeds a time limit. The notice circuit 3 records and holds the content of an SCSI bus state signal 103 when the overflow signal is received. Then, the notice circuit 3 informs the host computer 4 of the locking state of the SCSI bus 101 and restores the restoration of the SCSI bus 101.