We are developing a diffused light irradiation probe using a small diameter flexible plastic optical fiber and aiming at a photodynamic therapy in a narrow area by uniform light irradiation. In a narrow area, a direct transmission light irradiation probe could not manipulate the irradiation area, and a diffused light irradiation probe is required. The conventional diffused light irradiation probe made of quartz, has a hard tip, and could not be inserted into a curved path. We developed a light diffuser made by a small diameter plastic optical fiber and completed a small diameter and flexible diffused light irradiation probe.