PURPOSE: To mass-produce stiff honeycomb core materials without depending on the development system, by cutting to specified width, forming rugged surface and cutting to specified length to form honeycomb pieces while transferring plate or strip metal blanks, and adhering the adjoining protruding surfaces.

CONSTITUTION: A steel strip 11 from a roll 10 is slit to slit pieces 15 of the width equal to height Ho of honeycomb core material by means of a slitter 13. These pieces are worked in rugged surface by gears for plastic working 17 of a forming machine 16, and cut to the length Wo of lateral side of panel due to the movement of an upper blade 18 of a cutting machine 20, so that honeycomb pieces 21 are formed. These pieces 21 are sent out in the widthwise direction to a conveyor by a pusher 26, and are inverted by hooks which move up and down every second piece. The protruding surfaces of the pieces are cleaned and dried, and coated with primer, then dried again. Then, an adhesive is spread over such protruding surfaces by roll, and the pieces 21 are joined together by the specified number of pieces so that the adjoining pieces may be glued together. After hardening the adhesive, a honeycomb core material is formed.