To prevent the abnormal dropping of a housing yoke by breaking of the hydraulic hose of a cylinder for housing yoke up and down, or some other causes.

A housing yoke 5 carries out the cutting process of a workpiece W by moving up and down along a lowering preventive member 21, but in the abnormal lowering time of the housing yoke 5, a lowering preventive member locking part 27 provided to the housing yoke 5 is locked to the lowering preventive member 21, so as to prevent the abnormal lowering of the housing yoke 5. That is, the lowering preventive member locking part 27 is locked to the serrate locking projection of the lowering preventive member 21, to prevent the abnormal lowering of the housing yoke 5. When an abnormal condition is generated to a hydraulic circuit to drive the cylinder 19 for housing yoke up and down, a hydraulic cylinder 31 for locking connected in series is also stopped, and a stopper 27 is locked to the lowering preventive member 21, so as to prevent the abnormal lowering of the housing yoke 5.