A spindle unit includes a casing defining a spindle housing space, a spindle housed in the spindle housing space with a gap from an inner wall of the casing and has a first large-diameter part and a second large-diameter part separated from the first large-diameter part in the axial direction, a motor coupled to a base end part of the spindle, a high-pressure air source, and a valve between the air source and an air supply path. When the valve is opened to introduce high-pressure air into the gap and the motor is rotated and the rotation speed of the spindle becomes equal to or higher than a predetermined rotation speed, the controller closes the valve to interrupt supply of the high-pressure air into the gap and the spindle is rotatably supported by a dynamic pressure air bearing enclosed in the gap and is dragged by the spindle's rotation.