A reciprocating mechanism (6) includes a worm shaft (21) with a helical groove (21a) and a sliding member (22). The worm shaft (21) is disposed parallel to a spool shaft (15). The sliding member (22) moves back-and-forth along an axial direction of the spool shaft (15) by rotation of the worm shaft (21). The sliding member (22) includes a body member (22a) with a recess portion (22d) having an engaging member (22b). The engaging member (22b) includes an engaging portion (70), a first shaft portion (71) and a second shaft portion (72). The engaging portion (70) engages with the helical groove (21 a). The first shaft portion (71) is formed on an end portion adjacent the engaging portion (70) to have a diameter smaller than a diameter of the engaging portion (70). The second shaft portion (72) is formed on an end portion of the first shaft portion (71) that is remote from the engaging portion (70) to have a diameter smaller than a diameter of the first shaft portion (71).【課題】對於紡車式捲線器的往復移動機構，可抑制滑動構件的遊動。；【解決手段】擺動機構6，是具有：與捲筒軸15平行配置並形成螺旋狀溝21a的螺軸21、及沿著螺軸21朝前後方向移動的滑動構件22。滑動構件22，是具有：設有貫通凹部22d的本體構件22a、及前端部與螺旋狀溝21a卡合的卡合構件22b。卡合構件22b，是具有：前端部卡合於螺旋狀溝21a的卡合部70、及在卡合部70的基端部形成比卡合部70的基端部小徑的第1軸部71、及在第1軸部71的基端部形成比第1軸部71小徑的第2軸部72。
