In a navigation system, display screen handling PG 2032 prepares hierarchically a plurality of operation screens to receive the setting from the operator, and changes the operation screen displayed on display 208 according to the content of the operation to operation button group 207 by the operator, the operation screen under the display, and said layered structure. The sense amplifier includes a capacitor operative between a first voltage established by a first sense current flowing in a first direction and corresponding to an unknown logic state of the memory cell, and a second voltage established by a second sense current flowing in a second direction and corresponding to a known logic state of the memory cell. The predetermined time is shorter than the operating time of the microcomputer 34 determined by the discharge time of the backup capacitor when the backup capacitor is working normally, and is longer than the operating time of the microcomputer 34 determined by the discharge time of the backup capacitor when the backup capacitor is not working normally.