An apparatus and method for adjusting a desired expression section according to input bit values in a mobile communication system that expresses only a defined partial section of the input bit values in order to make the number of output bits be less than the number of the input bits, and maps bit values included in unexpressed sections to a specific value is provided. In the apparatus and method, a measurer divides possible output bit values into at least three sections, and measures output frequencies of output bits for the respective sections for a predetermined time. A controller adjusts the desired expression section, when an output frequency for a specific section is greater than output frequencies for other sections.