PURPOSE: To prevent an inadequate input from a key, by providing a mask register on a keying circuit, and determining whether an output signal of a key corresponding to information is sent out or not, in accordance with information written in the mask register.

CONSTITUTION: A mask bit is written in advance at a position corresponding to 1 of a code in the X direction and the Y direction of a key on which it is desired to inhibit an output, in an X direction mask register 6 and a Y direction mask register 7. When one key is depressed, the code in the X direction and the Y direction passes through a mask checking circuit 2 and whether an output of the key is inhibited contents or not is informed to an output signal controlling circuit 3. In case of the circuit 3 is inhibited in contents, the circuit 3 outputs a code of all "0" to an output bus X0 and Y0, and displays invalidity. Also, when an output of the key is effective, data of an X direction output bus Xi and a Y direction output bus Yi are outputted to an output bus Xo and a Y direction output bus Yo. Subsequently, it is converted as to its code by an encoder 4, written in a key code register 5, and outputted to the outside.