PURPOSE: To avoid the trouble in correcting errors at the time of power failure, by memorizing the correction value obtained at the correcting point in a mechanical switch, in addition to an absolute rotary encoder which measures the accumulated errors.

CONSTITUTION: A running input which is proportional to the run distance is added to the rotary encoder 13 from a measuring wheel 11. The output of the rotary encoder 13 is inputted to a correction operating circuit 16. The correction value which is the result of the operation is memorized in the mechanism memory switch 18. When a power source is turned OFF by the power failure and the like, the memories in a correction value memory 17, a compensation operating circuit 19, a compensation remainder memory 20, and the like are all erased. However, the memory in the memory switch 18 is not erased. Therefore, the compensation can be immediately performed by inputting the valve in the memory switch into an addition operating circuit 21.