To provide an automatic milling machine with a light weight, an easy handling, and an interchangeability.

An upper mortar 37 and a lower mortar 36 of which a lower surface and an upper surface slide-contact are coaxially arranged. The upper mortar 37 is unrotatably fixed and the lower mortar 36 is rotatably supported by a driving source 4 through speed-reduction means 30, 31a-31c, 32a, 32b. The upper mortar 37 is forcedly press-contacted with the lower mortar 36 by a resilient means 57 appropriately.