A cooling water passage structure includes a cooling water passage in an internal combustion engine, a water pump, and a communication member. The communication member has a cooling water communication passage to supply cooling water from the water pump to the cooling water passage. The cooling water communication passage includes a water pump housing, a first cooling water passage, and a second cooling water passage. The water pump housing contains the water pump. The first cooling water passage has a substantially triangular shape that widens from a vertex on an upstream side. The second cooling water passage includes a cylindrical passage extending in a direction orthogonal to an operating axis of the water pump. The second cooling water passage is displaced from an operating plane of the water pump.