PURPOSE: To make it possible to recognize all digits of the operation result at the first display time, by using the number of day display segments, which corresponds to the number of left digits of the operation result which cannot be displayed for the first display of double length display, for double length display.

CONSTITUTION: When ten-key 11 and function key 12 of key input part 10 are operated to perform operations, a prescribed operation is executed in operating circuit 9 through ROM address part 3, ROM4, instruction decoder 7, RAM5 provided with plural registers, and so on, and six digits and so on of the operation result transferred to the register of RAM5 are displayed on the numeric display part of display part 17 through display processing circuit 16. Meanwhile, the number of left digits such as three digits of the operation result is decoded through gate circuit 18 by decoder 19 which has the gate controlled through decoder 7, and the required number of segments of the day display part of display part 17 are lit to display left digits of the operation result, and thus, all digits of the operation result can be recognized for the first display of double length display.