The invention provides an improved seal structure of cutting fluid supplied to a spindle of a milling spindle. The milling spindle 100 comprises a spindle 20 supported within a housing via a bearing 16, and a drawbar 24 having a through coolant passage 22. The cutting fluid C1 passing through the rotary joint 40 and 50 is sent to the through coolant passage 22, but a portion thereof is collected in a drain chamber 105. A rotation member 110 is attached to a rear end portion of the spindle 20, which is equipped with a labyrinth device 150 having a lip seal member 152. An air supply member 120 has two air seal members 142 and 144, and air A1 is sent toward the labyrinth device 150, by which an air seal is formed.