In a machine tool or a processing method, a column base (12) is supported on a bed (11) movably in a horizontal direction, and a column (13) is installed vertically on the column base (12). A saddle (14) is supported on a side of the column (13) movably in a vertical direction, and a ram stock (15) is supported on the saddle (14) movably in a horizontal direction. A spindle (16) is rotatably supported by the ram stock (15), a damper (21) is provided on the ram stock (15), and an O-ring (22) is provided between the ram stock (15) and the damper (21), so that the O-ring (22) can be deformed by a pressing member (23). An NC device (24) can adjust a deformation degree of the O-ring (22) by the pressing member (23) depending on a mass of an attachment (17). Therefore, vibration of a tool can be effectively damped by changing a natural frequency according to replacement or movement of the attachment.