Cooling water passage structure for water-cooled type internal combustion engine

In a cooling water passage structure for a water-cooled type internal combustion engine, to ensure that the flow of cooling water in a hot water riser passage extending from a thermostat valve toward a carburetor can be assuredly caused and stopped without damaging the degree of freedom in laying out the passage and without complicating structure. ÄSolving meansÜ A cooling water passage structure comprising a cooling water pump (1) provided between a water jacket (3) for an internal combustion engine and a radiator (7), a thermostat valve (5) for controlling the quantitiy of cooling water circulated, and a by-pass passage (9) through which the cooling water is circulated without being circulated via the radiator (7) during when it is cold, wherein one end of a hot water riser passage disposed in parallel with the by-pass passage (9) and extending via a carburetor (11) is communicated with a port of the cooling water pump (1), and the other end is communicated with the thermostat valve (5) independently from an opening and closing portion of the thermostat valve for a port of the by-pass passage.