An optical fiber ribbon is capable of branching by means of any tool. The optical fiber ribbon is comprised of a plurality of optical fibers running in parallel, each of the optical fibers having an allowable radius of curvature; a blanket sheath totally covering the plurality of the optical fibers; one or more concavities formed at any one or more intermediates among the optical fibers; and slits respectively arranged in series at a regular interval along the concavities, the slits penetrating the blanket sheath and allowing the tool to be inserted and the blanket sheath to split by means of movement of the tool along the concavities. The length of each slit prior to insertion of the tool is so determined that flexures of the optical fibers induced by the tool widening the slits do not exceed the allowable radius of curvature.