JPS6249921B

Method and apparatus are disclosed for measuring the thickness of a workpiece by periodically transmitting ultrasonic pulses into the workpiece, receiving ultrasonic echo pulses reflected from the bottom surface of the workpiece, measuring the period of time elapsed from the time of transmission of each ultrasonic pulse to the time of reception of its related echo pulse by counting the number of clock pulses within the time period, counting each of such measured time periods by clock pulses, and deriving from each of such counted time periods and number of clock pulses within the time periods values a signal representing the thickness of the workpiece, wherein the measurement of the elapsed time period is effected with respect to a plurality of successive echo pulses, during which the phase of the clock pulses is caused to be shifted by a predetermined amount of 2 pi /N where N is an integer not less than 2 in a preselected direction upon completion of each measurement of the elapsed time period in such a manner that the resolution or accuracy in the thickness value finally obtained is improved.