PURPOSE: To eliminate the effect of the induced noises from an adjacent communication line, power line or the like and facilitate the movement of a scanning mechanism of an automatic flaw detecting system, by connecting a probe with an inspection room through an optical transmission path.

CONSTITUTION: A probe 10 is attached to a scanning mechanism 12 travelably connected with a guide rail 11 on the circumferential surface of a nozzle part 2 of a pressure vessel 1 and moved in the X and Y directions to scan the circumferential surface of the nozzle weld part. At a given position of the vessel 1, an optical coupler 19 which exchanges data and control signals with an optical controller 14 is placed and connected with an inspection room 20 through an optical cable 21. The echo signal from the nozzle weld part which is received by means of the probe 10 is inputted to a flaw detecting circuit 16 and led to an electro-photo converting circuit 18 together with the positional signal of the scanning mechanism 12. The signal outputted from the circuit 18 is propagated through the air and inputted to a data processing circuit 22 through the optical coupler 19 and the optical cable 21 in order to detect and display the existence and the size of a defect at the weld part.