PURPOSE: To secure a RAM area with high using efficiency by invalidating a write signal outputted from a control part when the control part outputs a write access within a range of values set up in two registers.

CONSTITUTION: Even when an error write access less than an address value set up in a register 102 and more than an address value set up in a register 103 is generated, write signals to respective RAMs 107 to 110 are held at invalid states. Thereby data stored in the RAMs 107 to 110 can be protected from being rewritten by a write access into an error miswriting protecting area. The protection area is determined by the values set up in the registers 102, 103. Thereby the miswriting protecting area with an optional size is set up on an optional address position by the minimum access unit of the control part 101 within the maximum access address range of the control part 101.