Abstract: An automatic tool changing device (ATC) for machining tools is disclosed. The ATC consists of arbor rotation guide means, arbor position changing means, arbor rotation means, tool changing means and ATC protection means. The ATC is positioned at a side of the movable table and effectively changes tools within the inner diameter circle of a rotary housing, thus shortening the arbor movement stroke and conserving the tool changing time during a tool changing operation. The ATC is also protected from the spindle stock of a machining tool, thus being free from any damage caused by chips from the spindle stock. The ATC is positioned at a side of the movable table in place of the column different from a typical ATC, so that it remarkably reduces the volume of the ATC protection cover and saves the manufacturing cost of such machining centers.