When left and right feed switches are pressed simultaneously for three seconds at the step S1, a special mode selection screen 1 is changed such that the name of each operating mode is displayed at a display portion corresponding to the position of each of switches <1> to <4>. Thus, pressing any of the corresponding switches <1> to <4> under each display makes it possible to select each operating mode. In this way, an operation mode can be easily and reliably selected.