A keyboard apparatus having mass bodies that pivotally move in accordance with a key operation that facilitates key attachment operations. In a key unit, a plurality of keys having driving parts are integrally connected by a common base end part in a condition in which the keys are pivotally movable. A plurality of mass bodies have driven parts disposed in correspondence to the driving parts of the keys, respectively, and are disposed in a pivotally movable condition with respect to a key frame having a securing part. In an assembled state in which the common base end part is secured to the securing part and the driving parts are engaged with the corresponding driven parts, respectively, in response to a pivotal movement of each of the keys, the corresponding driven parts are driven by the driving parts, respectively, so that the corresponding mass bodies pivotally move. By moving the key unit in a longitudinal direction of the keys in a state in which the mass bodies are disposed in the key frame, the driving parts of all of the keys of the key unit simultaneously engage with the corresponding driven parts, respectively, and when the driving parts are simultaneously engaged with the driven parts, the common base end part is positioned at a position in which the common base end part can be secured to the securing part.