PURPOSE: To attain high-speed scanning and high resolution, by rotating a scanning antenna consisting of a conical horn and a circular waveguide.

CONSTITUTION: A scanning antenna device 1 consisting of the conical horn and the circular waveguide is rotated around the center by a motor or the like. In case of, for example, the depth resolution of a three-dimensional image is required, a prime oscillator 10 outputs a signal which is subjected to frequency modulation with the trigger signal from a timing generator 11. Next, this signal is divided into two directions by a directional coupler 9, and one transmission signal is radiated from the scanning antenna through a circulator 6, and the other is used as a local signal of a detector. The scanning antenna 1 measures a scattered radio wave from an object by the trigger signal while the antenna 1 is rotating, thereby attaining an annular hologram data.