Abstract: An apparatus for measuring the thickness of a clad material having an outer mother metal and an inner clad metal. The apparatus includes a transmitter crystal and a receiver crystal of a double crystal angle-type probe which contact the outer surface of the mother metal, for receiving a first echo from the boundary surface of the mother metal and clad metal, and for receiving a second echo from the inner, bottom surface of the clad metal. The apparatus further includes an amplifier for amplifying the echo signals, a detector for detecting the zero-crossing points of the echoes, a zero point determining circuit, a calculator for calculating the periods from a zero point in time to the zero-crossing points, and a calculator for calculating the thickness of the clad material and clad metal based on the calculated periods.