A cooling water passage structure (10) for an internal combustion engine (1) provided with a supercharger (8), comprises a passage forming member (14) attached to a main body (2) of the internal combustion engine, and internally defining a branch passage (13) communicating with an outlet (12B) of a main body water jacket (12) formed in the main body of the internal combustion engine, and a water temperature sensor (80) provided in the passage forming member to detect a temperature of the cooling water flowing through the branch passage. The water temperature sensor is positioned in a part of a main supply passage (31) defined in the passage forming member closer to an inlet end of the main supply passage than a junction between the main supply passage and a supercharger return passage (34) is.