To provide a sizing warp having excellent size coating film and cohesion power and suitable for the weaving of a high-density woven fabric having excellent quality and provide a high-density woven fabric keeping stable high quality.

The sizing yarn for a high-density woven fabric has a cohesion power of ≥100 cycles measured by a cohesion power tester on the woven fabric. The cohesion power ratio of the sizing warp before and after weaving (after weaving/before weaving) is ≥0.70. The high-density synthetic fiber woven fabric produced by using the sizing warp has a total cover factor of the warp and weft on the gray fabric of ≥2,000.