Abstract: An ultra-directional speaker having a modulator 33 for modulating an ultrasonic carrier signal with an input electric signal from an audible sound signal source, and an emitter 44 for emitting an output of the modulator 33 is mounted in a moving object 1 having a target tracking system for sensing a target in a surrounding space in real time using the above-mentioned emitter 44. The moving object equipped with ultra-directional speaker can therefore transmit a voice only to a specific target through parametric action caused by the nonlinearity of finite amplitude of ultrasonic wave.