To prevent a clamped phenomenon of a band saw blade by arranging a feed controlling means feeding the band saw by the predetermined quantity in the reverse direction to the cutting feeding direction when a time counted result by means of a timer means reaches the predetermined time.

When a time counted result by means of a timer reaches a cutting continuing time T0 in a RAM, a direction controlling valve 22 is driven, pressure oil from a hydraulic pump 23 is fed to a lower side port in a hydraulic cylinder 5, a saw frame 4 is lifted for a time t0, and then, the direction controlling valve 22 is returned to the original cutting condition position again so as to lower the saw frame 4. If pressure data P reaches P0 again while the saw frame 4 is lowered again, the saw frame 4 is lifted for time t0 after a lapse of time T0 by the same way. These actions are repeated, and the saw frame 4 is lifted to the initial position when the saw frame 4 reaches the lower end and cutting is finished.