To prevent a supporting frame from being cut by a band saw blade by arranging a protection member, against which an edge point of a band saw blade can butt, in the supporting frame.

In installation of a band saw blade 9, a screw member is rotated in one direction and a driven saw blade wheel 21 is moved to the right so as to be brought close to a driving saw blade wheel 19, and the band saw blade 9 is wound on a pair of saw blade wheels 19, 21. In this process, a blade tip 9b of the band saw blade 9 is directed to the supporting frame 17. Subsequently, the band saw blade 9 is supported by means of a pair of saw blade guides 29, 31 freely movably in the traveling direction with a cutting action exhibiting part twisted. In this process, as the band saw blade 9 is twisted, the band saw blade 9 is moved to the rear side (supporting frame 17 side) by means of elastic force of the band saw blade 9, however, an edge point 9b of the band saw blade 9 collides against a collided part in a protection roller 35, so that the edge point 9b of the blade saw blade 9 is prevented from touching the supporting frame 17.