This paper describes a fiber optic oxygen sensor using phosphorescence and its application to clinical examinations. Since the quenching ratio of phosphorescence is proportional to oxygen partial pressure by the SternVolmer's formula, it is possible to estimate the oxygen concentration from the measured emission intensity. Combining an optical bundle with a luminescent material: Ru(bpy)32+/Nafion membrane, we have fabricated a luminous probe. This probe was experimentally investigated and was observed as having the characteristics of an oxygen concentration sensor in visible light. The probes were demonstrated to have certain advantages for example they can be operated in both liquid and gas phases. Also, they are stable to pH and flow velocities. As a clinical application, the probe can reliably measure oxygen concentrations of whole blood in vivo.