A water pump attachment structure of a water-cooled internal combustion engine is provided that can facilitate the maintenance of a valve train. A water pump attachment structure of a water-cooled internal combustion engine is as below. A semicircular cut-away surface (20u) of a water pump holder (20) is opposed to a semicircular surface (3u) of a cylinder head (3) to form a water pump support circular hole (21). Screw members (39b) are passed through respective screw through-holes (3h) of the cylinder head (3) and threadedly engaged with internal thread holes (20h) of the water pump holder (20). Thus, the water pump holder (20) is fastened to the cylinder head (3). A water pump (30) is fitted into and secured to the water pump support circular hole (21). A cylinder head cover (4) is attached to the cylinder head (3) by allowing their mating surfaces to conform with each other with the water pump holder (20) liquid-tightly sandwiched therebetween.