We proposed the method of obtaining a strong reflected light from a deep artery to expand the area where the oxygen saturation level can be measured. We fabricated the phantoms which imitated an optical characteristic of human body. An artery model was laid inside the phantom. We irradiated the light of a LED to the phantom, and measured reflected light from the artery model in the phantom by a photodiode. We obtained a strong reflected light from a deep artery model by adjusting the distance between LED and a photodiode. We conducted a similar experiment using the human body, and verified the efficacy of adjusting the distance between a LED and a photodiode.