PURPOSE: To improve the operativity and the use efficiency, by providing a diode between two terminals of a CPU and subordinating the switching of a key, by which plural functions are selected, to the switching of functions of the main body.

CONSTITUTION: A diode D is connected in such direction that a current is flowed from a terminal B toward a terminal A. When a switch SW1 is switched to a calculation mode-side terminal 2, a forward current is flowed to the diode, and the potential of the anode becomes approximately equal to that of the cathode. Thus, since "0" of the terminal 2 of the changeover switch SW1 is always transmitted to the terminal B independently of the state of a reverse/surface changeover switch SW2, a CPU is operated in accordance with the reverse key.