Plastics and metals are widely used in a variety of industrial products including automobiles, aircraft, and electronics. Plastics have several advantageous characteristics, including ease of forming complicated shapes, lightness, and low cost. Metals also have several advantages, such as high mechanical strength and good thermal conductivity. Conventionally, these materials are fastened by adhesive bonding or by a mechanical-joint such as a rivet or bolt. With adhesive bonding, significant issues including volatile-organic-compound (VOC) emission, long curing time, proper storage for adhesive resin, and quality control must be solved, while with mechanical bonding, separate mechanical-processing operations and joint component are required. Additional challenges are presented by the limited degree of design freedom and weight gain. In response to these issues, new joining process technologies, such as friction spot joining [1] and [2], ultrasonic metal welding [3], and laser direct joining [4], have recently been reported by several researchers. The advantages of laser joining include high precision and low distortion (since only the periphery of the target region irradiated by the laser is heated up rapidly in a concentrated manner), profitable formation of complicated three-dimensional shapes, the possibility of joints for large-scale products (since the laser is scanned), and a reduction in the cost of man-hours. Laser transmission welding of polymers is therefore widely researched as a promising technology for many industries