We propose a method to plan assembly motion of an object from a planned contact sequence which consists of vertex-triangle contact and edge-edge contact, where faces of polyhedral objects are triangulated. The distance functions are determined according to contact situation of a vertex-triangle contact or an edge-edge contact in order to approach the next contact state while maintaining the current contact and avoiding collisions. The triangulation enables reduction of search range for determining the distance functions. The motion of an object is optimized using differential distance functions until reaching the next contact state. We show through simulations of two assembly tasks that the proposed method can simply determine distance functions and plan the assembly motion.