operation shaft 22 vertically movably passes through the supporting piece 21 one end upper end in fig2 a and 2 b of the operation shaft 22 is pivotally connected to another end of the connecting lever 20 a supporting arm 23 is fixed to the bracket 15 and an intermediate portion of a l shaped intermediate lever 24 is pivotably connected to the supporting arm 23 the l shaped intermediate lever 24 has a rear arm portion 24 pivotably connected to another end lower end of the operation shaft 22 and has a bifurcated portion 25 engageable with the linking pin 14 protruding from the movable blade 9 further a pair of upper and lower adjusting nuts 26 and 27 are threadingly engaged with the operation shaft 22 at opposite sides with respect to the supporting piece 21 and a first coil spring 28 is disposed over the operation shaft 22 between the upper nut 26 and the supporting piece 21 a second coil spring 29 is also provided between the lower nut 27 and the supporting piece 21 by changing engagement positions of the adjusting nuts 26 and 27 biasing forces of the coil springs 28 and 29 can be controlled with the structure of the arm portion 1 when the both solenoids 17 and 18 are held in deenergized states the movable blade 9 has the standby or thread releasable position p1.