This paper presents a new topology optimization method that supports decision-making to obtain innovative designs at the conceptual design phase for mechanical structures. This method is developed based on structural and function-oriented elements such as frame and panel elements. For each of the frame elements, the rotational angle denoting the principal direction of the second moment of inertia is included as a design variable, and a way to obtain the optimal angle is derived from Karush-Kuhn-Tucker (KKT) conditions. For the panel elements, two types of panel element are introduced based the assumed stress method. Several numerical examples confirm the validity and utility of the proposed method, which should be useful for mechanical design engineers at the conceptual design phase. © 2005, The Japan Society of Mechanical Engineers. All rights reserved.