A technique of near-infrared (NIR) transillumination imaging was applied to develop an injection assist system. We have developed a simultaneous observation system for a body surface image and a transillumination image of blood vessels. The system consists of a high-intensity and low-leak light source, near-infrared CMOS camera and a small and light one-eye head mounted display. In transillumination imaging, we could visualize the veins in the adult forearm with the thickness of as much as 67 mm. A model phantom of the veins in the forearm was fabricated with the blood-contained tubes buried in pork thigh meat. The blood tube was hardly observable with a naked eye. Using the developed system, we could identify the blood tube in the NIR transillumination image, and could extract the blood successfully with the assist of the NIR image.