In the case where radar image data obtained from a radar device equipped in a flying body and optical image data of a district taken by the radar device are synthesized to produce a terrestrial object information judging image, the radar image data are approximated to a black and white panchromatic image character. The radar image data approximated to the black and white panchromatic character and the optical image data are aligned in position and then synthesized. As a suitable embodiment, in an approximation processing of the radar image data to the black and white panchromatic image character, histogram conversion processing is carried out in accordance with a histogram characteristic of the radar image data.