Abstract: A 1-bit microcomputer driven by a clock signal has first and second output ports. The signals from these output ports control first and second transistors whose output signals are supplied to step-up transformers. The output signals from these transformers drive the sounding devices constructed of piezoelectric elements to produce a chord sound. The memory of the microcomputer stores a program by which first and second square-wave pulse-train signals formed of a combination of a plurality of square wave pulses and in which each unit pulse train has a mutually different basic frequency are output from the first and second output ports.