Recently, simulation-based ergonomic assessments for handheld products, such as mobile phones, have seen a growing interest and have been increasingly studied. In these studies, the combination of 3D product models and "digital hands", which are a parametric 3D models of human hands, have been used. One of the keys to the ergonomic assessment using the digital hand is the "grasp synthesis" where plausible grasp postures for the product model have to be generated. In this paper, we propose a new database-driven grasp synthesis method considering the geometric constraints of grasping handheld products. The proposed method can generate more plausible grasp postures for handheld products in easier interactions than those of previous ones.