In a gear spindle, outer cylinder gear sections (11) are each integrally formed on an inner peripheral surface of a spindle outer cylinder and inner cylinder gear sections (14) are each integrally formed on an outer peripheral surface of a spindle inner cylinder (13). An oil seal (27) that seals in the lubricating oil (20) for each of the aforementioned gear sections includes, a seal body (29) having a channel-shaped cross section and interposed in the peripheral gap between the inner peripheral surface of the spindle outer cylinder and the outer peripheral surface of the spindle inner cylinder (13), and a seal mounting member (30) that includes a band, a spring, or the like that tightens and fixes the seal body to the outer peripheral surface of the spindle inner cylinder (13) to allow expansion of the seal body in the axial direction in the aforementioned peripheral gap.