PURPOSE: To avoid such a case where data are mistakenly rewritten due to runaway, etc., by allowing the write signal to a RAM via a switch only when the alteration of data is needed.

CONSTITUTION: A data rewrite allowing switch 4 is connected to the plus side Vcc of a DC power supply via a resistance 5 as well as directly to the minus side of the DC power supply. The joint between the switch 4 and the resistance 5 is connected to a 2nd input terminal of a microcomputer 1 and to a 2nd input terminal of an OR circuit 6. A 1st input terminal of the circuit 6 is connected to a write output terminal -WR of the computer 1 together with the output terminal of the circuit 6 connected to a write input terminal -WR of a RAM3. When the switch 4 is kept OFF, the 2nd input (a) of the circuit 6 is connected to the plus side Vcc of the DC power supply via the resistance 5 and therefore set at a high level. then the write output signal -WR is inhibited by the circuit 6 to the RAM3. As a result, the computer 1 is unable to rewrite the data stored in the RAM3.