PURPOSE: To obtain position information independent of a propagating state by receiving an ultrasonic wave generated form an ultrasonic wave generator located at the peripheral part of a display device by a pen with a built-in ultrasonic wave detector, and counting it by a counter, and executing arithmetic operation by a computing element.

CONSTITUTION: The ultrasonic wave generated at time t1 from the ultrasonic wave generator 2 located at the peripheral part of the display device 1 propagates on the surface of the device 1, and is detected at the time t3 by the ultrasonic wave detector 3 installed at the opposite side of the device 1, and a time interval k1=t3-t1 is counted by the counter 9. When an inputting operator touches the surface of the device 1 by the tip of the pen 4 with the built-in ultrasonic wave detector 5 at the time t2 between the time t1 and the time t3, the time interval k2= t2-t1 is counted by the counter 9. By calculating a relative position to the full scale of a display screen by dividing the interval k2 by the interval k1 by the computing element 7, the position information on the surface of the device 1 is obtained. Thus, the change of the propagating speed of the ultrasonic wave due to a reason such as temperature, pressure, and the deformation of the surface of the device 1, etc., is canceled, and always the correct position information can be obtained.