Abstract: An ultrasonic thickness gauge including a split-type probe to perform the transmission-reception of ultrasonic waves via a material board to be measured, a zero point calibration circuit to delay the zero point by a specified delay time from the wave transmission timing, and a measuring circuit to count clock pulses for the duration from the zero point to the wave reception timing. This ultrasonic thickness gauge is characterized in that it also includes a counter to count, within specified range, the clock pulses relating to the board thickness in each previous measurement conducted by using the preset zero point, and a zero point adjusting circuit to vary the zero point of the zero point calibration circuit in proportion to the count value of the above-mentioned counter.