PURPOSE: To form the belly surface of a Koto (Japanese traditional zither) by holding lumber by means of a pair of upper and lower jigs and fastening it by means of bolts and then heating and boiling it for bearing water into the core of a material and thereafter press-compressing and then fastening it again.
CONSTITUTION: An original lumber board is cut in accordance with a jig and after removing the outer skin 3, it is cut longitudinally and in parallel along the outer diameter in order to form an original material 2. After that, with a lower mold frame 4 having a protruded upper surface and an upper mold frame 6 having a recessed upper surface 7 which is faced with the upper part of the lower mold, the original material 2 is held therebetween and then a number of fastening bolts 16 are inserted into fastening holes 6, thus fastening them by means of nuts 17. While holding the original lumber between the upper jig 6 and the lower jig 4, the original material is borne with water sufficiently to its core and then heated wherein it is compressed rapidly by means of a press machine as being left in a framed state and after cooling, the jigs are removed therefrom.