PURPOSE: To prevent the digit errors due to increase of the number of digits by inputting each digit of the supplied numerical data through each digit input part.

CONSTITUTION: For instance, one of keys K1-K4 showing the numbers 1-4 is inverted to an H level from an L level in terms of its operating state. Thus the operating states of all keys showing the numbers smaller than that of the inverted key are inverted to H levels. When the H level is inverted to the L level, the operating states of all keys showing the numbers larger than that of the inverted key are inverted to L levels. These operating states are written into a key state holding part. Then a digit input part 6 turns on a light emitting part 2 of the key kept at an H level among those keys K1-K4 stored in the key state holding part. At the same time, the parts 2 of the keys kept in L levels are turned off. Thus it is possible to always handle the numerical data with a positional concept and to prevent the digit errors due to increase of the number of digits.