PURPOSE: To improve the operability of a small-sized unit by providing a belt driving unit-type folding knife device, the knife position of which can be controlled to move up and down by a control screw mechanism, with the knife device body not moved.

CONSTITUTION: In case of controlling the vertical position of a knife according to the crank motion, a fixing nut 25 is loosened, and a handle grip 23 is turned to rotate an adjust screw 24 so that a knife shaft 18 threaded on a cap screw portion 14 is, with the movement of the screw 24, adapted to move up and down with expansion and contraction of an elastic member 20 round a core bar 21, so that the height of the knife position can be controlled simply, quickly and accurately. Then, the fixing nut is fastened to be fixed. In this arrangement, only the knife shaft 18 can be directly fine-controlled quickly and correctly by simply turning a screw, not moving the main body (a) and regardless of the crank up- -and-down motion of the knife.