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