This paper introduces a system to draw a local map based on range data which are measured by ultrasonic sensors on a submersible vehicle. Topography is represented as the depth values at two-dimensional square cells. Measured depth values and their weights to represent their certainty at each cell are accumulated as the information about the obstacle occupancy and emptiness. The depth of seabed is conjectured with these values, or judged “unknown” when the amount of information is not sufficient. The features of the system are as follows : 1) Topography is conjectured by combining a large number of sensor readings from multiple positions although each reading has uncertainty of the point where the ultrasonic wave is reflected.2) The system offers the amount of information which represents the index of reliability of that depth value.3) The maximum depth for the vehicle to work safely is available when the depth of the seabed has not been conjectured.4) The path-plan is generated to survey the target area, which is more efficient than a prescribed path-plan such as simply tracing the orthogonal lines. Experiments at a testing pool and simulations on workstations demonstrate that the proposed system is significantly advantageous for detailed local mapping by a submersible vehicle.