Nozzle for producing glass fibers having modified cross-section, and modified cross-section glass fiber production device and production method therefor

This nozzle 5 for producing glass fibers having modified cross-section, is provided, at a leading end thereof where molten glass flows out, with: a pair of long wall parts 51 which are disposed opposite to each other in the X direction; a pair of short wall parts 52 which are disposed opposite to each other in the Y direction orthogonal to the X direction, and which have an X-direction dimension smaller than a Y-direction dimension of the long wall parts 51; and a flattened nozzle hole 53 defined by the long wall parts 51 and the short wall parts 52. Each of the long wall parts 51 is provided with a cutout 54 having a Y-direction opening width W that is gradually larger toward the leading end side.