To provide an air spindle unit suppressing air consumption and causing no galling phenomenon on the air spindle unit even when the pressure of supplied air is reduced.

The air spindle unit 36 includes a spindle housing 38 defining a thrust air bearing 42 and radial air bearings 40a, 40b, and a motor 44 connected to a spindle 26 and rotating the spindle 26, and includes an air distribution valve 56 distributing high-pressure air to the thrust air bearing 42 and the first and second radial air bearings 40a, 40b. The air distribution valve 56 supplies air only to the second radial air bearing 40b when an air pressure supplied from an air supply source 58 is equal to or larger than a predetermined pressure, and supplies air to the first and second radial air bearings 40a, 40b when a supplied air pressure is smaller than the predetermined pressure.