An optical fiber winding bobbin comprising: a main winding body portion that is configured such that an optical fiber is wound on an outer circumferential surface; a main winding flange that is provided on one end portion of the main winding body portion in an axial direction and that protrudes radially outward; a plurality of first ribs that protrude axially outward from an outer surface of the main winding flange, the plurality of first ribs are formed on the outer surface facing an axially outer side, wherein a thin portion, of which an axial thickness is smaller than an axial thickness of an inner circumferential edge portion connected to the main winding body portion, is formed at an outer circumferential portion of the main winding flange, which is positioned outward of a central portion in a radial direction, the optical fiber winding bobbin is characterized in that it further comprises: a slit that extends in the radial direction over an area from the inner circumferential edge portion to an outer circumferential edge of the main winding flange, the slit is formed in the main winding flange; a plurality of second ribs formed on first portions included in the portions of the outer surface, wherein the first portions are positioned adjacent to a radially inner end of the slit in a circumferential direction; a plurality of third ribs formed on second portions included in the portions of the outer surface, wherein the plurality of third ribs are positioned on a side opposite to the first portions on which the second ribs are formed in the radial direction, and a central axis of the main winding flange is interposed between the second ribs and the third ribs in the radial direction. a main winding body portion that is configured such that an optical fiber is wound on an outer circumferential surface; a main winding flange that is provided on one end portion of the main winding body portion in an axial direction and that protrudes radially outward; a plurality of first ribs that protrude axially outward from an outer surface of the main winding flange, the plurality of first ribs are formed on the outer surface facing an axially outer side, wherein a thin portion, of which an axial thickness is smaller than an axial thickness of an inner circumferential edge portion connected to the main winding body portion, is formed at an outer circumferential portion of the main winding flange, which is positioned outward of a central portion in a radial direction, the optical fiber winding bobbin is characterized in that it further comprises: a slit that extends in the radial direction over an area from the inner circumferential edge portion to an outer circumferential edge of the main winding flange, the slit is formed in the main winding flange; a plurality of second ribs formed on first portions included in the portions of the outer surface, wherein the first portions are positioned adjacent to a radially inner end of the slit in a circumferential direction; a plurality of third ribs formed on second portions included in the portions of the outer surface, wherein the plurality of third ribs are positioned on a side opposite to the first portions on which the second ribs are formed in the radial direction, and a central axis of the main winding flange is interposed between the second ribs and the third ribs in the radial direction. a slit that extends in the radial direction over an area from the inner circumferential edge portion to an outer circumferential edge of the main winding flange, the slit is formed in the main winding flange; a plurality of second ribs formed on first portions included in the portions of the outer surface, wherein the first portions are positioned adjacent to a radially inner end of the slit in a circumferential direction; a plurality of third ribs formed on second portions included in the portions of the outer surface, wherein the plurality of third ribs are positioned on a side opposite to the first portions on which the second ribs are formed in the radial direction, and a central axis of the main winding flange is interposed between the second ribs and the third ribs in the radial direction.