An optical unit is comprised of a plurality of optical base fibers twisted together without any member serving as a center of twisting so that each optical base fiber comes in contact with adjacent optical base fibers along the whole length, each of the optical base fibers including an optical fiber and a sleeve consisting essentially of silicone, a filler having the plurality of optical base fibers embedded therein, the filler consisting essentially of silicone, and a sheath covering the filler and the plurality of optical base fibers embedded in the filler.