To easily recognize the noise level by the visual monitoring by mounting a noise level display means on a joint part of the injection cylinder parts obtained by dividing an air injection cylinder into two along the axial direction thereof.

A noise level display means 40 is integrated in the injection cylinder parts 16a, 16b of an air injection cylinder 16 with the fitting of the left and right cover parts obtained by dividing a cover into two. The wind force by the injected air current which is sent by pressure and guided to the air injection cylinder 16 acts on a semi-spherical projecting part 46 of a movable plate 42 to move the movable plate 42 in the leeward direction against the energization force of a compression coil spring 45. By this movement, an indicating projection 43 mounted on a surface side of the movable plate 42 is moved in an opening part 41a of the noise level display plate 41, and stopped on the position where the energization force of the compression coil spring 45 and the wind force are balanced, whereby the noise level can be recognized by confirming a graduation 41b formed on an outer peripheral part of the opening part 41a corresponding to the indicating projection 43.