PURPOSE: To easily detect a radio wave beam sent by a radar antenna to be detected by controlling a scanning device for an antenna which performs a beam scan on a body and varying the scanning speed at random at a period of plural cycles.

CONSTITUTION: A PN generator 5 generates a random pulse signal, which supplies to a variable frequency power source 6. The power source 6 varies in frequency and supplies electric power to an AC electric motor 1. The electric motor 1 varies in rotating speed while following up variation in the frequency of the power source. The rotating speed is sent to the antenna 3 through a coupling mechanism 2 to vary the rotating speed of the scanning of the radio wave beam and the scanning speed is varied at random at a period of plural cycles.