A system, consisting of: a tool having a width X; and an optical fiber ribbon capable of being made to branch by means of the tool, the optical fiber ribbon comprising: a plurality of optical fibers running in parallel, each of the optical fibers having an allowable radius of curvature, R; a blanket sheath totally covering the plurality of the optical fibers; one or more concavities formed on the blanket sheath at any one or more intermediates among the optical fibers; and slits respectively arranged in series at a regular interval, B, 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, each of the slits having a length, A, prior to insertion of the tool so determined that flexures of the optical fibers induced by the tool widening the slits do not exceed the allowable radius of curvature, R) wherein each of the optical fibers has an outer diameter, D, the blanket sheath has a thickness, t1, from the respective optical fibers to a surface, each of the concavities has a thickness, t2, and the length, A, of the slits, the allowable radius of curvature, R, and the intervals, B, satisfy: Y = A < 500 ⁢ mm, Y < A + B ≤ 500 ⁢ mm, 0.01 ≤ A / A + B < 1, 0 < t 2 < D + 2 ⁢ t 1, and Y = 2 R 2 − R − X 2 2, wherein Y is a lower limit length of the slits. a tool having a width X; and an optical fiber ribbon capable of being made to branch by means of the tool, the optical fiber ribbon comprising: a plurality of optical fibers running in parallel, each of the optical fibers having an allowable radius of curvature, R; a blanket sheath totally covering the plurality of the optical fibers; one or more concavities formed on the blanket sheath at any one or more intermediates among the optical fibers; and slits respectively arranged in series at a regular interval, B, 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, each of the slits having a length, A, prior to insertion of the tool so determined that flexures of the optical fibers induced by the tool widening the slits do not exceed the allowable radius of curvature, R) wherein each of the optical fibers has an outer diameter, D, the blanket sheath has a thickness, t1, from the respective optical fibers to a surface, each of the concavities has a thickness, t2, and the length, A, of the slits, the allowable radius of curvature, R, and the intervals, B, satisfy: Y = A < 500 ⁢ mm, Y < A + B ≤ 500 ⁢ mm, 0.01 ≤ A / A + B < 1, 0 < t 2 < D + 2 ⁢ t 1, and Y = 2 R 2 − R − X 2 2, wherein Y is a lower limit length of the slits. a plurality of optical fibers running in parallel, each of the optical fibers having an allowable radius of curvature, R; a blanket sheath totally covering the plurality of the optical fibers; one or more concavities formed on the blanket sheath at any one or more intermediates among the optical fibers; and slits respectively arranged in series at a regular interval, B, 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, each of the slits having a length, A, prior to insertion of the tool so determined that flexures of the optical fibers induced by the tool widening the slits do not exceed the allowable radius of curvature, R) wherein each of the optical fibers has an outer diameter, D, the blanket sheath has a thickness, t1, from the respective optical fibers to a surface, each of the concavities has a thickness, t2, and the length, A, of the slits, the allowable radius of curvature, R, and the intervals, B, satisfy: Y = A < 500 ⁢ mm, Y < A + B ≤ 500 ⁢ mm, 0.01 ≤ A / A + B < 1, 0 < t 2 < D + 2 ⁢ t 1, and Y = 2 R 2 − R − X 2 2, wherein Y is a lower limit length of the slits.