Since working population is decreasing, industrial robots for various operations such as assembly are widely expected. Assembly sequences consist of low level tasks such as Peg-in-Hole Assembly, Snap Assembly, and so forth. In this paper, we propose a strategy of motion planning for manipulators that operate sash insertion into frame, a kind of assembly tasks. Sash insertion can be assumed as a three-dimensional path planning on the plane when an edge of the sash is always parallel to the frame. Motion of manipulator grasping the sash is generated from the planned motion of the sash by solving inverse kinematics. The generated motion for sash insertion is successfully performed by a 6 DOF manipulator and own-built sash and frame.