A system control device comprises a system LSI section having a plurality of functional blocks, a system control microcomputer section for controlling the control register of each of the functional blocks, an address decoding section for decoding an access address to a predetermined byte in a control register which the system control microcomputer section attempts to access, and issuing an access control signal to the whole of a single control register including the predetermined byte, an access control section for changing the access control signal to the whole of the single control register to an access control signal to the predetermined bytes of the plurality of control registers included in the system LSI section, with respect to access to an address to the predetermined byte, and an access mode control register for indicating whether or not the changing by the access control section is to be performed.