Whether or not motion compensation is necessary is determined for each of received radio wave signals acquired through observation on the basis of information on a difference between an planned trajectory and an actual trajectory of a platform (103), and a motion compensation process is performed on the received radio wave signals for which the motion compensation is determined to be necessary. An image generation process is performed on the received radio wave signals on which the motion compensation process has been performed and the received radio wave signals on which the motion compensation process has not been performed depending on the results of determination, so that a SAR image of an observation object is generated.