To provide a driving unit for a mobile element designed not to apply a load greater than a predetermined value( threshold value ) on the mobile element under reciprocating motion and designed to enable a threshold value to be set differently in its forward and backward motions.

The driving unit has the following construction and mechanism: A bracket 38 set movably along a guide rail 37 and a power transmission section 39 are joined each other in such a state that an engagement part 42b on the tip of the plunger 42a of a damper 41 fixed on the bracket 38 is engaged on a 1st engagement part 45b fixed on the power transmission section 39. The power transmission section 39 and a needle 36 for threading a slip-off-proof yarn are joined each other in such a state that an engagement part 47b on the tip of the plunger 47a of a damper 46 mounted on the power transmission section 39 is engaged on a 2nd engagement part 48 provided at a point toward the basal end of the needle 36. When a load is applied at a level greater than a predetermined value in the forward motion of the needle 36, the engagement part 47b and the 2nd engagement part 48 are disengaged, and when a load is applied at a level greater than a predetermined value in the backward motion, the engagement part 42b and the 1st engagement part 45b are disengaged.