PURPOSE: To make it possible to satisfactorily track a moving object by providing a selective output terminal for outputting a selective signal at the time point when the intensity becomes stronger by a predetermined amount or more than the strength of a course signal at the strength of displacement signal in comparing means.

CONSTITUTION: Branches 100, 200 have the same functions. The measured values of coordinates Dx↓.k and Dy↓.k along axes X, Y are received by measured value input terminals 1, 2. A linear line distance L1 is input to a filter 12. The actually passed distance L2 at the same time interval τ as the distance L1 is input to an absolute value circuit 21, and the absolute value is input to a filter 22. Further, the measured value Dx↓.k is input to a threshold 31, and the value is supplied to a filter 32. The outputs hx, fx, gx of the filters 12, 22, 32 are input to an arithmetic circuit 101, which outputs the mean measured value P of the moving efficiency at the two instantaneous times separated by the interval τcorresponding to a memory coefficient α. The maximum value circuit 300 obtains the measured value R of the displacement efficiency of the outputs Px, Py from the branches 100, 200.