Disclosed herein is a spinning nozzle having one or more discharge openings and one or more introduction openings located upstream the discharge openings for producing pitch-based carbon fibers characterized by the presence of one or more spiral members in each introduction openings. According to the nozzle of the present invention, part of the pitch which is introduced to the introduction opening flows down along the spiral member while the flow of the pitch is affected by the spiral member. The remaining pitch flows into the discharge opening without being in contact with the spiral member, and then both are spun after being mixed through the discharge opening to prepare carbon fibers having a random structure. Especially, the carbon fibers of higher performances can be prepared when a spiral having uneven outer diameters is employed.