PURPOSE: To provide a detector surely and quickly detecting a yarn breakage even when this occurs in a single fiber.

CONSTITUTION: Each yarn F is wound on a tubular rubber layer 3 by the rotations of each bobbin #1-#20 of an under net braider 8 to form under layer 4. The yarn breakage detector 11 is provided with a laser beam sensor 12 consisting of a light projector 16 and a light receiver 17, an amplifier 13, a digital meter relay 14 and a controller 15. When each yarn F passes through the space between the light projector 16 and the light receiver 17 of the laser beam sensor 12, the laser beam which is not reflected on the yarn is received by the light receiver 17. A timing signal and a yarn width signal proportional to the width of yarn F are output. Depending on the both signals, it is determined whether the yarn F has the width of larger than the specified size. If NG signals for an identical bobbin continues for specified times, it is decided that yarn is broken. When yarn F is clung itself and becomes narrow for a short period, it is not judged as yarn breakage.