PURPOSE: To scan plural key groups while digit signals are back to their start points by generating time slots more than the digit number of an external display means, permitting any one of them to apply an interruption and altering sequentially the key groups to be scanned.

CONSTITUTION: A digit signal generating circuit 2 built-in a microcomputer 1 decodes a digit information I1 and outputs a digit signal S1 to the display 3 of the external display means. A segment signal generating circuit 4 decodes a segment information I2 to output it as a segment signal S2 to the display 3, and in addition impresses the same signal as a key scanning signal S3 also on a keyboard 5. The digit signal generating circuit 2 has time slots more than the digit number of the display 3 by one, drives a data alteration timing circuit 7 with the aid of the time slot to generate an interruption, and causes the segment signal generating circuit 4 to generate different key scanning signals corresponding to the segment information I2.