the surface defect detecting apparatus for rotating cylindrical metal pieces which move in a lengthwise direction relative to a sensor block , has a plurality of eddy - current flaw detecting coils facing the test piece . the sensor block rotates about a horizontal axis extending vertically with respect to the axis of the test piece to follow up deflections of the test piece in the direction of the axis of rotation . front and rear ends of the sensor block , have a pair of guide wheels that engage the piece ; also present are color marking nozzles for marking defective areas in response to detector signals . relative movement in the longitudinal direction of the test piece is carried out by means of a carriage carrying the sensor block . each guide wheel is spaced from another in a plane transverse to the longitudinal direction of the test piece . a number of coils with designated impedances are connected to a generator and are disposed in longitudinal direction of the test piece in a single sensor holder within the sensor block . the holder has turning wheels at front and rear ends thereof in a longitudinal direction of the test piece , and are each positioned between the guide wheels in a direction for spiral scanning , and in rotatable contact with the outer surface of the test piece . impedances of the coils are selected to allow feedback amplifiers to form a feedback circuit with two of the coils and are linear for eddy - current signals corresponding to different flaw depth ranges . signals of a detector unit associated with different flaw depth range are amplified within the linear characteristic range and supplied to nozzles which spray different colors depending on flaw depth .