Receiving a request for canceling setting, a control circuit erases data stored in a corresponding block, changes a value of a protection flag, and cancels protection setting. When an overall protection is set for any block, the control circuit prohibits access to all blocks, except when it is an operation mode for activating a memory program contained in the microcomputer. Further, control circuit permits an access to a block M only when partial protection is set, CPU is in the mode for activating a memory program contained in the microcomputer and the access is for reading an instruction code in accordance with an instruction fetch.