PURPOSE: To detect a hair crack generated in a refractory material, specially, the lining refractory material in operation and its remaining thickness at the same time by applying a longitudinal ultrasonic wave and a lateral ultrasonic wave from the external surface of the lining refractory body and receiving its reflected wave, and performing the composition processing of the received waveform.

CONSTITUTION: The lateral ultrasonic wave applied to the refractory material 8 is reflected by the hair crack 13 when there is the hair crack 13 and detected by a receiving sensor 2. The longitudinal ultrasonic wave which is applied, on the other hand, is reflected partially by the hair crack, but the majority of it passes through it, so the receiving sensor 2 detects the thickness of the refractory material 8. The electric signal obtained by the receiving sensor 2 is amplified by an amplifier 6 and put together with respective transmission signals by a waveform processor 7. Consequently, the lateral reflected wave 15 indicates the distance to the hair crack in the refractory material 8 according to a surface wave 14 and the reflected wave 16 of the longitudinal wave indicates the remaining thickness of the refractory material 8.