To make a user easily discriminate keys, to reduce an erroneous using condition key selection and to reduce the temporal damages, physical damages and financial damages, etc., by variably controlling the display sizes of soft keys inside a display by the respective using frequencies of the respective soft keys outputted from a calculation means.

For instance, the center positions of the respective using condition soft keys 22, 23 and 25 and longitudinal and lateral lengths (a1, a2, b1, b2, c1 and c2) to the outer frames of the respective using condition keys are decided. Then, in the case where the using frequency of a certain using condition key from a calculating device exceeds 1/10 for instance of the using frequency of the entire using condition keys, the longitudinal and lateral lengths of the using condition key are changed to 1.25 multiple of an original state. Also, in the case that the using frequency of the using condition key is smaller than 1/50 of the using frequency of the entire using condition keys for instance, the size of the using condition key is changed to 0.8 multiple of the original state. In the case of changing the size of the using condition key, the center position of the using condition key is not changed.