PURPOSE: To selectively/inexpensively/highly reliably detect unidirectional force in the initial stage of generating a kickback by using an electric pulse by impact of a piezoelectric crystal body arranged in a blade attachment as a control device circuit releasing impulse.

CONSTITUTION: A nut 32 arranged on a screw 30 has the projecting part 33 inserted into a hole 34 in a saw tooth 36, and when the screw 30 is rotated, the nut 32 and the projecting part 33 move along a groove 35 of a crankcase end wall 29, so that the saw tooth 36 moves in the lengthwise direction. The inside end of the screw 30 contacts with a piezoelectric crystal body 37, the crystal body 37 contacts with the bottom of a hole 31, and an elastic flange 38 provided on the screw 30 is introduced into a circular groove 39 to prevent the screw 30 from moving in the hole 31. Impact at kickback time of the saw tooth 36 is transmitted to the crystal body 37, and when the crystal body 37 is pressed, voltage is generated between metallic plates 40, 41, and in a comparator 44 to receive reference voltage V through a conductor 45, since a signal of output 46 is supplied to a control electrode 47 of a thyristor 48 when the piezoelectric crystal body 37 becomes the reference voltage V or more, the thyristor 48 is electrically continued, and a brake is actuated since capacity 49 is discharged.