To efficiently perform cutting work by precisely preventing constriction of a sawtooth by improving constriction detecting accuracy.

A sawtooth position to a work is detected by a sawtooth position detecting device 33, and an actuating current value of a sawtooth motor 65 in respective sawtooth positions is detected by an electric current-detecting device 77. A preset actuating current pattern to a cutting position of the work is preset by an actuating current value in the respective sawtooth positions, and is stored in a storage device 85. An actual actuating current pattern and a preset actuating current pattern are compared with each other by a comparing judging device 89. When the actual actuating current value rapidly increases without staying in an allowable range of the preset actuating current pattern, the comparing judging device 89 precisely judges that a change in this actuating current value is caused by constriction, and commands constriction avoiding operation, and after a notch of a band saw blade is temporarily retreated, the notch of the band saw blade is again advanced, and the work is cut.