There is provided an irradiation device which irradiates light on an inner side face of a biological lumen. The irradiation device includes an optical fiber through which light passes, and a plurality of optical members each having a spherical shape. The plurality of optical members are arrayed in a line along a direction in which light emitted from a distal end of the optical fiber is radiated so that the light is radiated toward the inner surface of the biological lumen.
