A keyboard apparatus of an electronic keyboard instrument, which is compact in size and broad in a permissible movable range at the time of receiving a force exerting in a width direction of keys. The keyboard apparatus includes key main bodies each provided with a key-fulcrum function device permitting a key to be pivotable in the key depression/release direction and including a vertical hinge and a left-to-right hinge. The vertical hinge is formed into a thin plate-like piece member extending downward of the key and coupled to a common base end and the left-to-right hinge, which is formed into a thin plate-shaped piece member extending along the longitudinal direction of the key and coupled to a connecting portion that is formed into a plate shape extending downward of the key and coupled to the key main body.