A main spindle located in a region of a rotary guide bush device where the rotary guide bush device (has been attached to be disposed in a position of a front (F) along an axis (C) direction when the rotary guide bush device having a guide bush, which is supported in a removable manner by a guide bush-supporting base (supporting base) provided in the front (F) along the axis (C) direction of the main spindle, is removed. A seal member (fixed to the guide bush-supporting base has contact with an outer circumferential surface of a main spindle cover over an entire circumference, so that processing with or without the guide bush can be selected and a complicated process required for switching the processing with or without the guide bush can be simplified.