A hollow portion of a spindle through which a draw bar and a disc spring assembly are inserted has a closed chamber structure and constitutes an oil pool chamber in which a lubricating oil is filled. The bearing portions that support the draw bar in the oil pool on both sides have the same inner diameter so that the volume of the oil pool chamber does not change when the draw bar is moved forward and backward. As a result, the lubricating oil is not pushed out of the oil pool chamber. Thus, a spindle unit can be provided in which a clamping device including the draw bar and the disc spring assembly for clamping a tool or a workpiece pallet at the forward end of the spindle can be reliably lubricated, so that the friction on and the breakage of the clamping device is reduced and that the service life of the unit is lengthened.