Abstract: An apparatus which measures a thickness of a layer using transverse waves of ultrasonic waves, and includes: a sensor unit having a probe for obliquely transmitting and receiving to and from the surface of a material to be measured having first and second layers with different acoustic impedances in a depth direction: an extractor for extracting, from a wave reception signal representing waves received by the probe, reflected waves from a boundary between the first and the second layers of the material; and a calculator for calculating a distance between the surface of the material and the boundary.