A gear attachment structure receives rotation of a handle assembly. The gear attachment structure includes a traverse cam shaft and a driven gear. The traverse cam shaft includes a shaft body, a gear support portion and a gear engaging portion. The gear support portion includes intersecting helical grooves on the outer peripheral surface. The gear support portion is disposed on an end of the shaft body and has a circular cross-section. The gear engaging portion includes a pair of circular-arc portions and a pair of straight portions. The circular-arc portions are opposed to each other while being either disposed along and within the outer periphery of the gear support portion or disposed radial outwards of the gear support portion. One of the straight portions is coupled to one ends of the circular-arc portions whereas the other of the straight portions is coupled to the other ends of the circular-arc portions.