A cooling water passage structure of internal combustion engine includes a cylinder-block-side cooling water passage, a cylinder-head-side cooling water passage, a heat exchanger, and cooling water passage pipes. The cylinder-block-side cooling water passage is formed inside a cylinder block of an engine to cause cooling water to flow through. The cylinder-head-side cooling water passage is formed inside a cylinder head to cause cooling water to flow through. The heat exchanger is configured to dissipate heat of cooling water to external air to decrease a temperature of the cooling water. The cooling water passage pipes couple the heat exchanger and the engine to exchange cooling water. The cylinder head includes a cooling water inlet. The cooling water passage pipe is coupled to the cooling water inlet. The cooling water from the heat exchanger flows into the cooling water inlet.