PURPOSE: To easily detect a ghost wave source without complicated operation by adopting one unit of receiving antenna.

CONSTITUTION: The detector is provided with a receiving antenna 1, a PDUR meter 4 which detects the level ratio of a desired wave to a reflection wave and the delay time of the reflection wave against the desired wave based on the receiving signals from the antenna 1, a voltage level measuring device 5 which measures orientation patterns in a horizontal plane based on the receiving signals from the antenna 1, and a CPU 7 which specifies the coming direction of the reflection wave based on the already-known horizontal orientation characteristic, level ratio, delay time, and orientation characteristic of the antenna 1 and calculates the distance between the antenna 1 and the reflection source of the reflection wave.