A portable electronic keyboard musical instrument comprises a casing comprising a pair of plastic-molded upper and lower shells coupled together. A plurality of keys are mounted on the upper shell in juxtaposed relation. The upper shell includes an integral fulcrum portion formed internally thereof and pivotally supporting each key, an integral stop means for limiting the downward and upward movements of the keys, an integral guide means for guiding the downward and upward movements of the keys, and an integral mounting portion for supporting a key switch structure. The stop means, the guide means and the mounting portion are disposed in underlying relation to the juxtaposed keys.