PURPOSE: To make possible the automatic deflashing of a non-circular type core by pressing a roller cutter fitted loosely to a cutter shaft to the inside circumferential surface of a core bore by centrifugal force and rotating the cutter along the inside circumferential surface of the bore.

CONSTITUTION: The roller cutter 24 which is provided with a blade 24b on the outside circumferential surface and is tapered in the lower part is loosely fitted to the cutter shaft 22 and the shaft 22 is attached to the bracket 21 of a revolving shaft 11a driven by an air motor 1. The water jacket core (a) set to a positioning jig 26 is fixed by the retaining jig 15 of a core retaining plate 13 and the bracket 21 inserted into the core (a) is rotationally driven. The cutter 24 rotates to deflash the core (a) while the cutter is held pressed to the inside surface of the bore (b) of the core (a) by the centrifugal force. The automatic deflahsing of the non-circular type core of the water jacket is thus made possible.