PURPOSE: To provide the bus interface device which enables one arbitrary processor to occupy and use a shared device connected to plural processors.

CONSTITUTION: Processors 4A and 4B share a shared device 1. When the processor 4A or 4B requests access to the shared device, a bus controller 3 controls buffers 2A and 2B so that one processor can write or read data in or from the shared device 1. When the processor 4A will occupy and use the shared device 1, it writes its own identifier in a discrimination register 11. The bus controller 3 limits access to the shared device 1 from the processor 4B while the identifier of the processor 4A is written.