PURPOSE: To detect high-temp. cracks and the positions thereof non-destructively in real time, by moving master and slave ultrasonic sensors having prescribed relative positions with a welding torch integrally with said torch and detecting and deciding weld defects.

CONSTITUTION: A welding torch 3 and a master sensor M are fixed in relative positions in such a way as to correspond to the timing when the temp. range vulnerable to generation of defects is attained in the cooling stage of weld zones according to welding speeds. Slave sensors S1, S2 are provided respectively before and behind the sensor M, and the sensors M, S1, S2 move integrally with the torch 3. The ultrasonic detection outputs by the sensors S1, M, S2 are applied to a comparator 11 for time difference, and the arrival sequence of signals is compared. The detected output of the sensor M is taken in as a defect detection signal only when the output detected by the sensor M is faster than the detection outputs of the sensors S1, S2. Said signal is evaluated in an evaulation circuit 11 for the degree of harmness and the position is recorded with a recorder 7 and is marked with a marker 16, whereby the high-temp. cracking by welding and the position thereof is detected non-destructively in real time.