PURPOSE: To shorten time required for starting a system without being affected by a memory extending amount.

CONSTITUTION: When a power source at the main body of a computer is turned on, at the same time, a diagnostic processing part 5 disconnects a memory device 3 from a system bus 7 by controlling a bus connector/disconnector 2. The diagnostic processing part 5 generates an address to the memory device 3 while controlling an address generator 4 and executes the series of diagnostic processing or initialize processing to the designated address on the memory device 3. When the series of diagnostic processing or initialize processing to the memory device 3 are finished, the diagnostic processing part 5 connects the memory device 3 to the system bus 7 by controlling the bus connector/ disconnector 2 and finishes the processing.