A white key for a keyboard musical instrument comprises a key body made of resin in the form of a hollow box shape having an upper wall to be pressed by an instrument player and vertical walls extending downward from the periphery of the upper wall. The upper wall of the key includes a wide part and a narrow part, the narrow part providing a space to accommodate a black key. The upper wall has a thickness not exceeding 1.5 mm and the wide part has a projection extending from the upper wall downward. The projection may be in the form of a rib bridging the vertical walls or of columns extruding from the lower surface of the upper wall. The frequency band of a vibration mode due to a deformation of the upper wall caused when struck by the player's finger is shifted higher when compared with the case where no projection is provided.