1. A design aiding device, comprising: a first storing section that stores board data created for a board that is to be designed; a second storing section that stored part data created for a part to be designed and arranged on the board; and a processor that creates assembled model data, representing an assembled model that assumes arrangement of the part on the board, based on the board data and the part data stored in the first storing section and the second storing section, respectively, said processor including a recognizing section that recognizes a mounting face of a part model defined by the part data, the mounting face being mounted on a board model defined by the board data; and an arrangement processing section that creates the assembled model data by arranging the part model on the board model based on the mounting face recognized by the recognizing section which determines a first reference plane including a face of the part model having a maximum surface area and a second reference plane and a third reference plane which are perpendicular to the first reference plane and which are perpendicular to each other; sets six planes which are each parallel to one of the first through the third reference planes and which contact one with each of six outermost planes of the part model; obtains shapes of the outermost planes on the six planes; and recognizes one of the six outermost planes as the mounting face on the basis of the shapes of the six outermost planes and information about an arranging face of the board model to arrange the part model, the information being included in the board data.