PURPOSE: To facilitate air supply to a plurality of different loads by providing a switch valve which supplies pressure air for a main spindle air curtain through a second pressure controller to reduce the pressure air for the main spindle air curtain, and directly supplies pressure air for a main spindle tapered hole air blow without passing through the second pressure controller.

CONSTITUTION: A switch valve V1 is always closed during the machining of a work, an pressure air sent from a pressure air source E is supplied to a gap between a main spindle 11 and a spindle head 1 through a common pressure controller 2 and a second pressure controller 3 to set up the reduction or pressure for a main spindle air curtain as main spindle air curtain air. The pressure air thereby forms an air curtain jetting constantly from a gap between the main spindle and the main spindle head 1. The switch valve V1 is closed during the changing of a tool, and the pressure air A is directly supplied to a main spindle tapered hole without passing through the second pressure controller 3 as air for main spindle tapered hole blow.