1. A target object gripping apparatus comprising: an estimation unit configured to estimate an orientation of a target object based on orientation estimation parameters; a gripping unit configured to grip the target object based on the orientation of the target object estimated by said estimation unit; a detection unit configured to detect a failure of gripping by said gripping unit; and a modifying unit configured to modify the orientation estimation parameters based on the orientation of the target object when said detection unit detects a gripping failure.