A system controller 16 determines whether the quality of either a distance image or a light intensity image satisfies reference quality, and, when the quality of either the distance image or the light intensity image dos not satisfy the reference quality, outputs a speed change command to lower a speed, an altitude change command to change a submarine altitude (depth), or the like to a navigation control unit 2, thereby changing a physical relative relation between a measurement plane 3 and a device in question. As an alternative, the system controller changes a beam scanning rate f, a beam divergence θ, or the like within limits at which the amount of variation in a spatial resolution does not exceed a permissible amount.