PURPOSE: To eliminate restrictions on the cutting length of a workpiece, obviate necessity for returning a band saw and make overall construction simple by disposing in a laterals direction the sides of both straight advancing parts of an endless band saw so hung on a pair of pulleys as to be capable of circulation.

CONSTITUTION: The side 7a of the straight advancing part 7 of a band saw 5 hung on a pair of pulleys 3 is arrayed with a guide roller 8 in a line toward a width direction and the passage 9 of the straight advancing part 7 is formed in the same line within an identical plane. When a workpiece 'W' is cut, therefore, the straight advancing part 7 at both use and non-use sides is arrayed in a line and as a trailing line passes through the groove of the workpiece 'W' cut with a leading line, the straight advancing part 7 at the non-use side does not come in contact with a part of the workpiece 'W' when said part 7 at the use side is employed for cutting. Consequently, cutting length becomes free from restrictions. Also, the band saw 5 is curved toward the direction of the thickness thereof, thereby restricting the return of the saw 5 for the formation of said part 7. Therefore, the fatigue of the band saw 5 is alleviated and overall construction can be made simple.