Ultrasonic waves from a transmit device are transmitted to an object subject to measurement. A surface reflected wave propagated in a contact medium and reflected at the surface of a tube and bottom surface reflected waves passing through an oxidation layer on the tube and reflected at the inner surface of the tube are convened into electric signals. A feature extracting part 5, in consideration of a correlation relationship of energy with respect to the frequency of a received signal, based on the received signal of ultrasonic waves converted to electric signals, extracts a signal feature data determined beforehand which has a strong correlation relationship with the thickness of a measured part. Based on the extracted signal feature data, a thickness conversion part references a relationship storage part which has stored therein beforehand a function that shows a relationship between the signal feature data and the thickness of the oxidation layer, and obtains the thickness of the oxidation layer.