<p>An apparatus for measuring a thickness of plate material comprises: a plate material whose thickness should be measured; a vibration pen to generate a plate wave by applying a vibration to the plate material; a vibration sensor which is disposed at an arbitrary position on the plate material and detects the plate wave which has propagated on the plate material; an extracting circuit to extract different frequency components of the plate wave detected by the vibration sensor; a timer to measure arrival times to the vibration sensor of the signals of the different frequency components extracted by the extracting circuit; and a controller to obtain a thickness of the plate material on the basis of the arrival times measured by the timer and the frequency components corresponding thereto. The extracting circuit is a band pass filter. <IMAGE></p>
