This paper presents a study on a design methodology for lightweight automatic transmission parts for vehicles. To improve fuel economy of the vehicle, weight reduction is a primary issue when designing automobile parts. Topology optimization, a particularly flexible type of structural optimization method, is an effective tool for developing lightweight designs. However, topology optimization methods available in most commercial software often provide optimization results that include grayscale areas, which cause the designs to be impractical from an engineering and manufacturing standpoint. To overcome this difficulty, we apply a level set-based topology optimization method that yields clear optimal configurations and therefore facilitates further improvement of the product design by engineers. In addition, we consider the imposition of a uniform cross-section constraint to improve the manufacturability of the obtained optimization results. This paper includes numerical examples in which the level set-based topology optimization method is applied using parking pawl model of AISIN AW automatic transmission. The optimization results demonstrate that the presented method provides clear optimal configurations. We only consider a maximum stiffness problem under volume constraint here from a weight reduction standpoint to verify a future potential of the method for the design of actual products.