An optical fiber is capable of radiating light radially in a wide range. The optical fiber includes an elongate primary coated optical fiber adapted to transmit light radiated for medical treatment. The primary coated optical fiber includes a core at a central portion and a cladding covering the core, and has an irradiation groove along the circumferential direction thereof. The irradiation groove extends from the cladding to reach the core of the primary coated optical fiber, and has side portions on both sides with respect to the axial direction of the primary coated optical fiber and a bottom portion. The side portions each have a protruding portion shaped to be protrude toward the inside of the irradiation groove.
