A keyboard musical instrument with a transpositional function includes a keyboard, a mode selection switch, a microcomputer, a musical tone signal generator and a sound system. The keyboard has a plurality of keys respectively representing different notes in a musical scale. The mode selection switch selects one of a transposition mode and a play mode. The microcomputer includes a CPU, a ROM, and a RAM to detect first and second depressed keys when the transposition mode is selected by the mode selection switch. The microcomputer generates transposition data having a value corresponding to an interval between the notes represented by the first and second keys. The microcomputer generates a note signal representing a note higher or lower by the interval represented by the transposition data than a note of a given depressed key when the play mode is selected by the mode selection switch. The musical tone signal generator generates a musical tone signal in response to the note signal and the sound system sounds a musical tone corresponding to the musical tone signal, so that the musical tone is a transposed one of a tone represented by the given depressed key by the interval.