PURPOSE: To appropriately update the memory of results of integration in a non- volatile memory element by memorizing an integrated running distance into the second memory area of the non-volatile memory element each time the voltage supplied from a DC power source falls.

CONSTITUTION: When the voltage supplied from a DC power source 10 falls for some reasons, a microcomputer 110 writes the current integrated running distance into the second memory area of a non-volatile memory element 100 as data B through a voltage drop detection circuit 80 to update the memory while the computation is halted. As the voltage restores, the computer 110 computes the absolute value X=A-B of data memorized in the first and second memory areas of the memory element 100. When X<10km, the memory contents of the second memory area are judged to be correct and it is generated as display command signal while when X≥10km, the memory contents of the second memory area are judged to have an error and the memory contents in the first memory area is memorized into an RAM while it is generated as display command signal. Thus, there will be no error accompanied in the computation and the display contents afterward.