Provided is a gear spindle that is optimal for reducing diameter, that is capable of maintaining adequate strength and a high torque transmission capacity, and that maintains the sealing properties of an oil seal and prevents said oil seal from being damaged during operation at a high rotational speed. In order to achieve this, outer cylinder gear sections (11) are each integrally formed on the inner peripheral surface of a spindle outer cylinder ((10), more specifically a large-diameter cylinder section (10a)) and inner cylinder gear sections (14) are each integrally formed on the outer peripheral surface of a spindle inner cylinder (13), and an oil seal (27) that seals in the lubricating oil (20) for each of the aforementioned gear sections comprises: a seal body (29) that has a channel-shaped cross section and that is interposed in the peripheral gap between the inner peripheral surface of the spindle outer cylinder ((10), more specifically the large-diameter cylinder section (10a)) and the outer peripheral surface of the spindle inner cylinder (13); and a seal mounting member (30) that comprises 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) such that expansion of the seal body in the axial direction is possible in the aforementioned peripheral gap.