PURPOSE: To prevent the contents of a storage device from an unfair writing by counting an address to be written next from a writing address executed in the past and information concerning a writing data length and when the writing address is discrepant to the output of a counting means, suppressing the writing.

CONSTITUTION: When an instruction to instruct the writing to a main storage device 2 is executed by a central processing unit 1, a main storage writing request is issued through a control line (ST request) 5. In the same way, a writing head byte address is sent through an address line 3, writing data are sent through an ST data line 6 and an effective writing data byte length is sent through a data line 4 and transferred to MS. In this case, by adding a writing address executed immediately before and a writing data length, the address to be counted next is counted and next, the justice of the writing to occur next is decided. When the destruction due to an unfair program is not intentionally executed, the possibility that the unfair writing address is coincident to a counting address is small.