Abstract: A method of detection of defects in a manufacturing process of an elongated metallic material (1) which manufacturing process is accomplished by rolls (3, 6) which rolls (3, 6) reduce the cross section of the elongated metallic material (1) thinner and/or divert or guide the path of the elongated metallic material (1). The formation and/or existence of defects of the elongated metallic material (1) is detected by sensing acoustic emission transmitted by origination or advance of the defects and the detection is accomplished by at least one acoustic emission (AE) sensor (9) having direct or indirect contact with the elongated metallic material (1) which AE sensor (9) transduces the sensed acoustic emission to electric signals and the signals are received by an analyzing unit (21) which is capable of detecting the indication and reporting time coded indication and/or the indicative amplitude of the originated or advanced defects.