A keyboard assembly for playing music automatically comprises: a plurality of juxtaposed keys including white keys and black keys, each being supported swingable for depression and release thereof; a plurality of juxtaposed swing weights, each for each of the keys and each being supported swingable as interlocked with the corresponding key; and a plurality of actuator devices, each for each of the swing weights to actuate the swing weight, which in turn drives the interlocked key to swing to its depressed position. The actuator devices are arrayed alternately in two rows with the arraying pitch between the actuator devices for the adjacent swing weights made different from the arraying pitch between the swing weights so that the actuator devices for the keys of C through E can be disposed closer to the C end and the actuator devices for the keys of F through B can be disposed closer to the B end, thereby securing a fitting space between the E actuator device and the F actuator device. Alternatively, the actuator devices may be equally spaced within the octave, while the swing weights are spaced unequally.