To provide a steering tube support structure for a bicycle having simplified and improved locking safety.

This steering tube support structure is constituted of the head tube 1 of a bicycle frame, a steering tube 2 extended from a front fork, and a bearing structure 5 rotatably supporting the steering tube 2 in the head tube 1. The steering tube 2 is provided with the surface having no screw thread. The bearing structure 5 is provided with the first race 13 firmly connected to the head tube 1, the second race 17 fitted to the steering tube 2, and a locking ring pressing the second race 17 in the axial direction toward the first race 13 and firmly connecting the second race 17 to the steering tube 2. The second race 17 is provided with a neck portion surrounding the steering tube 2 and capable of elastically fastening it. The locking ring is provided with an inner conical surface capable of being pressed in the axial direction toward the corresponding outer conical surface of the neck portion to fasten the neck portion to the steering tube 2 and firmly connect the second race 17 to the steering tube 2.