To make it possible to easily and accurately cut and to eliminate the labor-hour for the operation by providing a simple constitution.

A timber 61 is sandwiched fixedly between vice blocks 25a and 25b. The slit 62 of the depth L9 is cut by coaxially disposed two rotary cutting blades (Figure B). Then, the blocks 25a, 25b are retracted, the interval of the two blades is sequentially approached by the thickness of the blade and the blades repeatedly cut the timber. Eventually, to cut the residue 47 retained by the limit of the approach of the two blades, the timber 61 is laterally moved by the thickness of the blade, and the one blade cuts the timber. Thus, the slit 62 and a central space 63 are formed.