To make surely recognizable the actual bearing of a vehicle by setting the corresponding output voltage of a relative bearing detecting means as the neutral potential when the output voltage values of both filter means becomes the same as each other and correcting the drift error based on the potential.

Low band components of output voltage of a relative bearing detector 12 are passed through LPF 31, 32 to give first and second output voltage values of respectively different delay time. The voltage difference of the first output voltage and the second output voltage is made to be an absolute value by a subtractor 33 and an absolute value circuit 34 and the value is compared with the reference voltage REFa by an analog comparator 35 to produce a first comparison result signal DCLR. A digital comparator 38 compares the count value DCNT of an oscillator 36 and a counter circuit 37 with a reference value REFd to produce a second comparison result signal DSTBL. Based on the signal DSTBL, the output of a potential signal DDLY obtained by digital conversion of the second output voltage is controlled by a latch circuit 40 and the potential signal DNULL from the latch circuit 40 is set to be the neutral potential of the relative course detection part 13 and CPU 41 correct the drift voltage based on the neutral potential.