An input device, including two or more ultrasonic transducers (UTs) driven by a driving signal (DS) for respectively sending a transmitting signal to sense an object; a driving circuit unit (DCU) generating the DS; a receiving circuit unit (RCU) determining whether echo signals (ESs) respectively returned from the two or more UTs are greater than a threshold value; a diplexer receiving and transmitting the DS to the two or more UTs, or receiving and transmitting the ESs to the RCU; and a control unit controlling the DCU to generate the DS, controlling the operation of the diplexer, and obtaining a relative motion information between the object and at least one of the two or more UTs according to all of ESs greater than the threshold value and determined by the RCU, so as to correspondingly output an input signal, is provided.