Abstract-We present a novel filtering method for building a grid map using sonar sensors. Recognizing that conflicting information from sonar sensor readings is the main reason of poor map quality, we established a proposition that put the conflicting information in order. Based on the proposition, we effectively filtered sonar readings that cause conflict. In addition, some information of filtered readings were not discarded but reprocessed to enhance the quality of the map. We call this filtering and reprocessing operation the conflict evaluation method. We evaluated the proposed method by experiments under various conditions.