PURPOSE: To provide an apparatus for automatically, effectively and early detecting a surface flaw of a plug for perforating a seamless steel tube such as a heat fused damage, etc.

CONSTITUTION: 'A system for scanning a surface of a plug with a laser luminous flux to detect a surface flaw of the plug for perforating a seamless steel tube' is applied as a surface flaw inspecting apparatus for the plug for perforating the tube. The apparatus comprises a light emitting unit for the laser luminous flux 13 to be scanned in a longitudinal direction on a surface of a laterally placed plug, a laser light receiver 6 for receiving a reflected light from the surface of the plug, a position detector 8 for detecting the longitudinal direction of the plug to be irradiated with the flux 13, a controller 10 for calculating.controlling a light receiving angle of the receiver 6 upon reception of positional information from the detector, and an image processor 12 for deciding a fused damage upon receiving of a light reception signal from the receiver.