PURPOSE: To improve cutting efficiency by applying the reversed circular cutting motion to a band saw before the rear end of its cutting operation portion reaches the side face of a cut material so as to prevent the actual cut length from being made shorter than the length of the operation portion.

CONSTITUTION: The length (l) of the cutting operation protion of a band saw 24 circularly cutting a cut material via the seesaw motion starts to be decreased when a point P reaches the side face Ws of the cut material and becomes zero when a point CP' reaches the side face Ws. When the length (l) of the cutting operation portion starts to be decreased and a point F reaches a contact point CP' the seesaw motion of the band saw 24 is stopped, then the band saw is applied with the reversed seesaw motion along the circular face CS' thus there is no state that the band saw is not entirely operated essentially, and cutting efficiency is improved.